Bump actions/checkout from 2 to 2.3.4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 2.3.4.
- [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...v2.3.4)

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

.github/workflows/check-links.yml

@@ -10,7 +10,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v2.3.4
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: