Set `UV_LINK_MODE=copy` for Windows test runs (#8350)
Cherry-picked from #8347
Might fix https://github.com/astral-sh/uv/issues/6940 — I'm not seeing a
failure over there after this change. I think there may be some problem
with concurrent reads of junctioned files on the DevDrive? It's really
hard to say.
We might lose some important test coverage with this change. I'm not
sure what to do about that either.
