As suggested by @samypr100 on #6680:
https://github.com/astral-sh/uv/pull/6680#issuecomment-2313607984
## Summary
Instead of using `UV_INTERNAL__TEST_DIR`, it simply exports `TEMP` when
running Windows jobs.
## Test Plan
I'm going to run this manually under ProcMon on my Windows machine and
see where uv writes temp files, hopefully to the dev drive and not
`%(LOCAL)APPDATA%` or something.
I'm going to commit a dummy code change and look at build time changes
in CI.