ci: linkage monitor CI update to stop using .kokoro/linkage-monitor.sh (#1718)

* ci: linkage monitor configuration to

synthtool's change expects the ci.yaml to use the Linkage Monitor
GitHub Actions. (It deletes .kokoro/linkage-monitor.sh)
#1717
However this repository excludes ci.yaml being managed by synthtool.
Therefore we need to manually maintain ci.yaml to stop using
linkage-monitor.sh.

* ci: install dependencies for Linkage Monitor

Author: suztomo

.github/workflows/ci.yaml

@@ -75,7 +75,11 @@ jobs:
         java-version: 8
     - run: java -version
     - run: .kokoro/install_dependencies.sh
-    - run: .kokoro/linkage-monitor.sh
+    - name: Install artifacts to local Maven repository
+      run: .kokoro/build.sh
+      shell: bash
+    - name: Validate dependencies with regard to com.google.cloud:libraries-bom (latest release)
+      uses: GoogleCloudPlatform/cloud-opensource-java/linkage-monitor@v1-linkagemonitor
   lint:
     runs-on: ubuntu-latest
     steps: