Attempt to use reflinks by default on Linux (#18117)
Copy of https://github.com/astral-sh/uv/pull/17753 which GitHub
auto-closed.
This adds test infrastructure for cross-device links and file systems
with reflink support. In short, we create a few extra file systems on
the CI runners then provide their paths to the test suite using
environment variables to ensure we have coverage. If the variables are
not set, the tests are skipped.
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Co-authored-by: Claude <noreply@anthropic.com>
