[ThinLTO] [test] cache.ll: Prevent Spotlight indexing of the output dir

The test output files whose atime is altered in the test were getting
accessed by Spotlight indexing on macOS, causing them to get an updated
atime and leading to the test not behaving as expected.

Reviewed By: jhenderson, steven_wu

Differential Revision: https://reviews.llvm.org/D84700