more fine-grained `--cache-check-probability`

[Alizter]

In Dune we have --cache-check-probability which when provided gives a way of invalidating the cache a certain amount of time aiding with reproducibility. As code bases scale, it becomes more unwieldy to allow cache invalidation to happen anywhere.

For example, one might have a bunch of tests that flake. With the cache enabled, it becomes impossible to observe, however setting --cache-check-probability will likely cause slow rebuilds elsewhere, making it unsuitable for observing just those tests.

It would be nice if we were able to have a more fine-grained version of --cache-check-probability so that certain directories can have the probability applied. This would likely be a sensible addition to the (env) stanza.

This would be useful in dune for our flaky cram tests allow developers to see the issues more often locally.

[rgrinberg]

This proposal describes that is better solved by more direct means. For example, by running the flaky test under more stress to trigger the flakiness. If we had something like that, would this feature still be needed? I doubt it.