Bump actions/checkout from 4.2.0 to 4.2.1

Bumps [actions/checkout](https://github.com/actions/checkout) from 4.2.0 to 4.2.1.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@d632683...eef6144)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/CI.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout
-        uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938
+        uses: actions/checkout@eef61447b9ff4aafe5dcd4e0bbf5d482be7e7871
       - uses: cachix/install-nix-action@3715ab1a11cac9e991980d7b4a28d80c7ebdd8f9
       - name: Build
         run: nix-shell --pure --run 'make SPHINXOPTS='-W' html'