[llvm] [llvm-exegesis] Fix race condition in subprocess mode (PR #72778)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Will keep an eye on the clang AVX512 Linux builder after this change to make sure the transient failures don't pop up again. If they do, I'll probably revert and do some more investigation.

https://github.com/llvm/llvm-project/pull/72778