gha-ci: instruct bazelisk to use Bazel 7.x

I noticed that our GitHub Actions CI started failing recently.
This change fixes the issue by sticking to Bazel 7.x.

Change-Id: Iab8b27512e526c2b204421b83a87241827c4aad4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/659495
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: stapelberg

.github/workflows/test.yml

@@ -28,6 +28,6 @@ jobs:
         key: ${{ runner.os }}-${{ hashFiles('integration_test.go') }}
     - name: Test
       env:
-        # Protobuf 25 does not yet support Bazel 7
-        USE_BAZEL_VERSION: latest-1
+        # Protobuf 30 does not yet support anything newer than Bazel 7
+        USE_BAZEL_VERSION: 7.x
       run: go test -run='^TestIntegration$' -v -timeout=60m -count=1 -failfast "$@"