[Github] Make Bazel Build/Test use GCS Cache

A bucket was added in llvm/llvm-zorg#650. Wire
it up in the job so we can actually take advantage of it.

Author: boomanaiden154

.github/workflows/bazel-checks.yml

@@ -52,4 +52,6 @@ jobs:
         working-directory: utils/bazel
         run: |
           bazelisk test --config=ci --sandbox_base="" \
+            --remote_cache=https://storage.googleapis.com/$CACHE_GCS_BUCKET-bazel \
+            --google_default_credentials \
             @llvm-project//llvm/unittests:adt_tests