chore: Refactor Github Action per b/485167538 (GoogleCloudPlatform#1377)

Co-authored-by: Ben Knutson <benknutson@google.com>

Author: google-admin

.github/workflows/coverage.yml

@@ -59,8 +59,8 @@ jobs:
 
       - name: Verify code coverage. If your reading this and the step has failed, please add tests to cover your changes.
         run: |
-          echo "BASE BRANCH CODE COVERAGE is ${{ env.CUR_COVER }}%"
-          echo "PULL REQUEST CODE COVERAGE is ${{ env.PR_COVER }}%"
-          if [ "${{ env.PR_COVER }}" -lt "${{ env.CUR_COVER }}"  ]; then
+          echo "BASE BRANCH CODE COVERAGE is ${CUR_COVER}%"
+          echo "PULL REQUEST CODE COVERAGE is ${PR_COVER}%"
+          if [ "${PR_COVER}" -lt "${CUR_COVER}"  ]; then
             exit 1;
           fi