Fix Java and Bazel version when building with Bazel in GitHub Actions.

This CL fixes the version of Java to JDK8 and Bazel to 3.7.1

For Java, ubuntu-latest will soon be switching from Java 8 to Java 11 as the default (https://github.com/actions/virtual-environments/issues/1816), and our Bazel builds currently don't work with Java 11 since compile_testing depends on @local_jdk//:lib/tools.jar which doesn't exist in Java 11.

https://github.com/google/bazel-common/blob/master/third_party/java/compile_testing/BUILD#L32

For Bazel, ubuntu-latest currently uses Bazel 4.0.0 by default, which fails with Robolectric tests @Config annotation:

```
error: missing required annotation argument: <clinit> @Config
```
RELNOTES=N/A
PiperOrigin-RevId: 358062883
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 328d4b1..958ca52 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -10,7 +10,17 @@
   bazel-test:
     name: 'Bazel tests'
     runs-on: ubuntu-latest
+    env:
+      # Our Bazel builds currently rely on JDK 8.
+      USE_JAVA_VERSION: '8'
+      # Our Bazel builds currently rely on 3.7.1. The version is set via
+      # baselisk by USE_BAZEL_VERSION: https://github.com/bazelbuild/bazelisk.
+      USE_BAZEL_VERSION: '3.7.1'
     steps:
+      - name: 'Install Java ${{ env.USE_JAVA_VERSION }}'
+        uses: actions/setup-java@v1
+        with:
+          java-version: '${{ env.USE_JAVA_VERSION }}'
       - name: 'Check out repository'
         uses: actions/checkout@v2
       - name: 'Cache local Maven repository'