Do not fail to hard link and produce a warning

[tcely]

#51 22.37 + HOME=/tmp/root XDG_CACHE_HOME=/cache PYTHONPYCACHEPREFIX=/cache/pycache uv --no-config --no-progress --no-managed-python pip install --strict --system --break-system-packages --requirements /cache/requirements.txt

#51 24.22 warning: Failed to hardlink files; falling back to full copy. This may lead to degraded performance.
#51 24.22          If the cache and target directories are on different filesystems, hardlinking may not be supported.
#51 24.22          If this is intentional, set `export UV_LINK_MODE=copy` or use `--link-mode=copy` to suppress this warning.