ci: actions/checkout: 2.3.4 -> 2.4.0

Bumps [actions/checkout](https://github.com/actions/checkout) from 2.3.4 to 2.4.0.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2.3.4...v2.4.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/build.yml

@@ -36,7 +36,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v2.3.4
+      uses: actions/checkout@v2.4.0
     - name: Install nix
       uses: cachix/install-nix-action@v14
       with: