Bump actions/checkout from 3 to 4 (#500)

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

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

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

.github/workflows/main.yaml

@@ -44,15 +44,15 @@ jobs:
     steps:
       # Boilerplate
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Install TeX Live
         uses: zauguin/install-texlive@v3
         with:
           # List the required TeX Live packages in a separate file to allow reuse in
           # different workflows.
           package_file: .github/tl_packages
       - name: Checkout fonts for testing
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           repository: zauguin/fontspec-test-fonts
           path: fontspec-test-fonts