fix: suppress uv hardlink warning with UV_LINK_MODE=copy

When running as a GitHub Action, the uv cache and target .venv directory
end up on different filesystems (container overlay vs host mount), causing
uv to emit a noisy warning on every run. Setting UV_LINK_MODE=copy tells
uv to use copy mode directly, suppressing the warning.

Signed-off-by: Zack Koppert <zkoppert@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: zkoppert

Dockerfile

@@ -19,5 +19,6 @@ HEALTHCHECK --interval=30s --timeout=10s --start-period=10s --retries=3 \
   CMD python3 -c "import os,sys; sys.exit(0 if os.path.exists('/action/workspace/evergreen.py') else 1)"
 
 ENV PYTHONUNBUFFERED=1
+ENV UV_LINK_MODE=copy
 CMD ["/action/workspace/evergreen.py"]
 ENTRYPOINT ["uv", "run", "--no-dev", "--project", "/action/workspace"]