Bump actions/checkout from v1 to v2.3.4

Bumps [actions/checkout](https://github.com/actions/checkout) from v1 to v2.3.4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v1...5a4ac90)

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

.github/workflows/tests.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Set up Git repository
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2.3.4
 
     - name: Set up Ruby
       uses: actions/setup-ruby@v1