ci: install pathspec into a venv on the runner

System Python on ubuntu-latest is PEP 668 externally-managed, so the
bare `pip install pathspec` failed the new CODEOWNERS-request step with
`externally-managed-environment`. Create a venv under /tmp and install
into it instead.

Signed-off-by: Matt Liberty <mliberty@precisioninno.com>

Author: maliberty

.github/workflows/github-actions-on-label-create.yml

@@ -97,8 +97,10 @@ jobs:
         UPSTREAM: ${{ env.UPSTREAM_OWNER }}/${{ env.UPSTREAM_REPO }}
       run: |
         set -euo pipefail
-        python3 -m pip install --quiet pathspec
-        python3 <<'PY'
+        # System Python on the runner is PEP 668 externally-managed; use a venv.
+        python3 -m venv /tmp/codeowners-venv
+        /tmp/codeowners-venv/bin/pip install --quiet pathspec
+        /tmp/codeowners-venv/bin/python <<'PY'
         import base64, json, os, subprocess, sys
         from pathspec import GitIgnoreSpec