lint: upgrade lint scripts for worktrees

Add a ci/lint.py script to run the linter both locally or inside the CI
(replacing .github/ci-lint-exec.py) which supports running from a
worktree.

Determines whether we are in a worktree, and mounts the real `.git`
directory as a read-only volume if we are.

.github/workflows/ci.yml

@@ -681,4 +681,6 @@ jobs:
           cache-provider: ${{ needs.runners.outputs.provider }}
 
       - name: CI script
-        run: python .github/ci-lint-exec.py
+        run: |
+          git worktree add ../lint-worktree HEAD
+          ../lint-worktree/ci/lint.py