build: only run commit message checks on PRs

Splits out the commit message check so it only runs against pull requests. If an invalid
commit has made it into the main branch, we can't do much about it anymore so
the failure generates unnecessary noise.

Author: crisbeto

.circleci/config.yml

@@ -336,7 +336,14 @@ jobs:
             yarn check-entry-point-setup $(bazel info bazel-bin)/entry_points_manifest.json
 
       - run: yarn ng-dev format changed --check << pipeline.git.base_revision >>
-      - run: yarn ng-dev commit-message validate-range --range << pipeline.git.base_revision >>...<<pipeline.git.revision>>
+      - run:
+          name: Check Commit Message (PR Only)
+          # Only run the commit message checks on pull requests since we can't do
+          # much about any failures that have made it into the main branch.
+          command: |
+            if [ -n "$CIRCLE_PR_NUMBER" ]; then
+              yarn ng-dev commit-message validate-range --range << pipeline.git.base_revision >>...<<pipeline.git.revision>>
+            fi
       - run: yarn ownerslint
       - run: yarn stylelint
       - run: yarn tslint