Fix copilot-setup-steps: relocate cache/tmp dirs to RUNNER_WORKDIR, add HOME

[Copilot]

The copilot-setup-steps.yml workflow was using github.workspace for pip/uv cache and temp directories, which caused failures on runners where the workspace may be read-only or reset between steps. The HOME variable was also unset, breaking tools that rely on it.

Changes

• Environment variables — moved PIP_CACHE_DIR, UV_CACHE_DIR, TMPDIR, XDG_CACHE_HOME, and HOME to point under RUNNER_WORKDIR (persistent, writable across steps); removed the step-level UV_CACHE_DIR override that was redundant
• Directory pre-creation — added explicit mkdir -p for all cache/tmp dirs at the start of the venv step so they exist before any tool tries to write to them
• pip --cache-dir flag — passed explicitly to all pip install calls to ensure the relocated cache is honoured regardless of env variable resolution order
• Path quoting — consistently quoted all shell variable expansions ("$GITHUB_WORKSPACE", "$GITHUB_PATH", etc.) to guard against paths with spaces
• --no-cache-dir → explicit cache dir — replaced the earlier --no-cache-dir workaround with a proper writable cache location

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.