RuntimesCacheTest.LookupWithManyStaleRuntimes is flaky

[danakj]

Description of the bug:

[ RUN      ] RuntimesCacheTest.LookupWithManyStaleRuntimes
toolchain/driver/runtimes_cache_test.cpp:644: Failure
Value of: stale_runtimes_0.base_dir().Stat()->unix_inode()
Expected: isn't equal to 2143098
  Actual: 2143098 (of type unsigned long)

Failure: https://github.com/carbon-language/carbon-lang/actions/runs/18323266557/job/52181416679?pr=6168

What did you do, or what's a simple way to reproduce the bug?

No response

What did you expect to happen?

No response

What actually happened?

No response

Any other information, logs, or outputs that you want to share?

No response

[zygoloid]

Another example: https://github.com/carbon-language/carbon-lang/actions/runs/18507797463/job/52740937965?pr=6058

[zygoloid]

I'd guess what's happening is that the inode of the old cache entry is being reused for the new one. I wonder if we could check the mtime rather than the inode to make this more reliable?