How did Job and Noah know that they should make sacrifices to God?

The sacrifice system was received after Exodus and is described in Leviticus.

It's true that it was recorded by Moses in Leviticus, but there is no reason to believe that sacrifices didn't exist before that time, nor even that there wasn't some form of priesthood.

Genesis records that sacrifices were being offered right from the beginning:

Abel also brought of the firstborn of his flock and of their fat.
And the LORD respected Abel and his offering,
— Genesis 4:4 (NKJV throughout)

Then Noah built an altar to the LORD, and took of every clean animal and of every clean bird, and offered burnt offerings on the altar.
— Genesis 8:20

Then Melchizedek king of Salem brought out bread and wine; he was the priest of God Most High.
— Genesis 14:18

Then Abraham lifted his eyes and looked, and there behind him was a ram caught in a thicket by its horns.
So Abraham went and took the ram, and offered it up for a burnt offering instead of his son.
— Genesis 22:3

Then Jacob offered a sacrifice on the mountain, and called his brethren to eat bread.
And they ate bread and stayed all night on the mountain.
— Genesis 31:54

What Moses did was to make a formal written record of how the Levitical priesthood should administer sacrifices.

This may have included new procedures and ceremonies specific to the covenant with Israel, but it would also have included the knowledge of sacrifices to God that had previously been passed down as traditional practice.

The same applies to God's commandments:

And I will make your [Isaac's] descendants multiply as the stars of heaven; I will give to your descendants all these lands; and in your seed all the nations of the earth shall be blessed; because Abraham obeyed My voice and kept My charge, My commandments, My statutes, and My laws.
— Genesis 26,4–5

It was God himself who instituted sacrifice :

Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them. [Genesis 3:21 KJV]

The source of the skins is not revealed. Therefore the implication is that a source is to be revealed in the future.

After this event, Abel follows the example :

And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering: [Genesis 4:4 KJV]

Following the divine example, Abel sacrificed of his flock and brought the fat. And God respected that Abel had taken note of His own action and had followed that example.

Noah follows the precedents set previously.

Job also follows these precedents.

These are obedient, faithful people who notice what God has done and who notice what their forebears have done and they humbly follow the way that has been shown them.

Argument from silence like "well, probably God did tell them somehow" is not a good argument.

This is incredibly biased. Absence of evidence is not evidence of absence; indeed, that claim is a well-known fallacy, which you are deliberately invoking.

Fortunately, we have plenty of positive evidence as well. In fact, God is explicitly recorded as talking to Adam (Genesis 2-3), Eve (Genesis 3), Cain (Genesis 4), Noah (Genesis 7-9), Job (Job), and many others. The idea that the only information God communicated to men is exactly what is recorded is simply absurd, and we see Cain and Abel offering sacrifices, and Noah doesn't seem to need to be told about clean and unclean animals.

So... why is this information missing?

A better question to ask is why do we expect it to be present? When telling a story, you generally don't fill in background information your audience already knows. It seems quite clear that people before Moses knew God, and knew at least some of what he expected in terms of laws and/or religious practice.

Another consideration is that Moses is generally credited for writing Genesis. That isn't to say that Genesis is fiction; rather, Moses took prior sources and compiled them into what we have today. Given that this likely happened at about the same time Leviticus was written (or at least at a time when Moses knew that Leviticus would be written), it's entirely plausible that he would have considered rehashing laws, practices and customs predating Leviticus to be superfluous.

It's true that Scripture is intended for us, today, but it was also intended for its contemporary audience... and for pre-Incarnation Jews, explaining the need and reasons to offer sacrifices would be as gratuitous as explaining the need and reasons to consume food or beverage. Thus, to them it would not be surprising that sacrifices are mentioned only occasionally, just as eating and drinking are mentioned only occasionally. Nor would it be any more surprising for no reason to be stated than for every instance of eating or drinking to omit "because he was hungry" or "because she was thirsty"; that information would be self-evident to the reader... just as it should be self-evident to us because we have the context of all of Scripture from which to infer such things. (It should be noted that there are many places in which understanding Scripture requires background knowledge; Genesis is hardly unusual in this respect.)

At any rate, the idea that God had not yet revealed Himself is clearly nonsense. The argument that people prior to Moses could not have known information that is not recorded prior to Leviticus is simply, and overtly, fallacious. It makes no sense when Scripture is considered in its original context, and it makes no sense when considered in our context today. In both cases, the "missing" information is either well known, or trivially inferred, from other sources.