Disable code coverage report in PRs for external users (#7484)

Author: granluo

.github/workflows/test_coverage.yml

@@ -275,6 +275,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Access to Metrics Service
+        if:  env.metrics_service_secret
         run: |
           # Install gcloud sdk
           curl https://sdk.cloud.google.com > install.sh
@@ -291,7 +292,7 @@ jobs:
         with:
           path: /Users/runner/test
       - name: Compare Diff and Post a Report
-        if: github.event_name == 'pull_request'
+        if: github.event_name == 'pull_request' && env.metrics_service_secret
         env:
           base_commit: ${{ needs.check.outputs.base_commit }}
         run: |
@@ -302,7 +303,7 @@ jobs:
           swift run CoverageReportGenerator --presubmit "firebase/firebase-ios-sdk" --commit "${GITHUB_SHA}" --token $(gcloud auth print-identity-token) --xcresult-dir "/Users/runner/test/codecoverage" --log-link "https://github.com/firebase/firebase-ios-sdk/actions/runs/${GITHUB_RUN_ID}" --pull-request-num ${{github.event.pull_request.number}} --base-commit "$base_commit"
           fi
       - name: Update New Coverage Data
-        if: github.event.pull_request.merged == true
+        if: github.event.pull_request.merged == true && env.metrics_service_secret
         run: |
           if [ -d "${{steps.download.outputs.download-path}}" ]; then
           cd scripts/code_coverage_report/generate_code_coverage_report