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/ci.yml

@@ -24,7 +24,7 @@ jobs:
           - java: 11
             os: ubuntu-latest
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2.3.4
     - uses: olafurpg/setup-scala@v10
       with:
         java-version: "adopt@1.${{ matrix.java }}"