We previously wrote our new cache file into a temporary directory and
the (atomically) renamed it to the canonical cache file path. This
rename operation unfortunately only works when both files are on the
same file system, and that's not always the case (e.g. when $TMPDIR is
on its own file system).
Instead, we can use Zig's AtomicFile to safely perform this operation
inside of the cache directory.
There's a new risk of a crash leaving the temporary file around in this
directory (and not getting cleaned up like $TMPDIR-based files), but the
probability is low and those files will only be readable by the creating
user (mode 0o600).
There's a new test cash that verifies the expected AtomicFile clean up
behavior. I also switched the file-oriented tests to use testing.tmpDir
rather than using our application-level TempDir type.
5423d64c6a