Make cacheFile thread-safe using file locks

[crtschin]

This was mentioned on on Matrix.

Changes include:

• Adds a cacheFileIn that allows varying the cache directory used, overriding the default behavior of using $XDG_CACHE_HOME.
• Made the above function thread-safe by using file-locks, and atomic file writes.

As long as downstream consumers were using the srcHash parameter as a content addressed tag, I don't think they have to necessarily use cacheFileIn and can keep using cacheFile as is after this change. I exported cacheFileIn mainly for the test I added.

[fendor]

I think hie-bios caches need to be created from scratch for the tests in HLS. Otherwise, we risk the tests using existing caches (e.g., on a rerun), changing the test run. But we want the tests to be run the same way every time!

While the lock might be useful regardless, e.g. on windows this might be a thing that helps with multiple processes trying to open a file, I think we need to be able to specify the XDG_CACHE_HOME without having to read the global XDG_CACHE_HOME variable. I.e., I think we should add a parameter to runCradle, such as CradleRunConfig (completely made up name), which allows us to optionally set the cache root directory.

[crtschin]

Otherwise, we risk the tests using existing caches (e.g., on a rerun), changing the test run. But we want the tests to be run the same way every time!

Fair! Do the last two commits correspond to what you had in mind? I kept the previous API-exposed function that default to the environment usage. I can drop those if you don't think they're useful to keep.

[fendor]

Yes, this is what I had in mind!

I think this is what we want. All the FilePaths are making me dizzy though, perhaps a newtype CacheDir = CacheDir FilePath is appropriate?

[fendor]

LGTM, just one improvement to the testsuite, then this is ready to merge :)

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