build: only run snapshot tests against main branch (#22424)

Limits the various snapshot tests to only run against the main branch. They are very likely to main against the older branches, because by definition Angular's latest changes at HEAD might not be backwards-compatible.

(cherry picked from commit 5f00365)

Author: crisbeto

.circleci/config.yml

@@ -118,25 +118,25 @@ var_18: &setup_snapshot_builds
     name: "Setting up Angular snapshot builds"
     command: node ./scripts/circleci/setup-angular-snapshots.js master
 
-# Filter to skip a job on builds for pull requests.
-var_19: &skip_on_pull_requests_filter
-  branches:
-      ignore:
-        - /pull\/\d+/
-
 # Filter which ensures that jobs only run for pull requests.
-var_20: &only_on_pull_requests_filter
+var_19: &only_on_pull_requests_filter
   branches:
       only:
         - /pull\/\d+/
 
 # Anchor for a step that notifies Slack when preceding steps failed.
-var_21: &slack_notify_on_failure
+var_20: &slack_notify_on_failure
   run:
     name: "Notifying team about job failure"
     when: on_fail
     command: node ./scripts/circleci/notify-slack-job-failure.js
 
+# Branch filter that only matches the main branch.
+var_21: &only_main_branch_filter
+  branches:
+      only:
+        - master
+
 # -----------------------------
 # Container version of CircleCI
 # -----------------------------
@@ -696,7 +696,7 @@ workflows:
           requires:
             - build_release_packages
       - ngcc_compatibility_snapshot:
-          filters: *skip_on_pull_requests_filter
+          filters: *only_main_branch_filter
           requires:
             - build_release_packages
       - publish_snapshots:
@@ -711,9 +711,12 @@ workflows:
       # Note that we need additional jobs for the cronjob snapshot tests because there
       # is no easy way to detect whether a job runs inside of a cronjob or specific
       # workflow. See: https://circleci.com/ideas/?idea=CCI-I-295
-      - snapshot_tests_local_browsers
-      - view_engine_snapshot_test_cronjob
-      - mdc_snapshot_test_cronjob
+      - snapshot_tests_local_browsers:
+          filters: *only_main_branch_filter
+      - view_engine_snapshot_test_cronjob:
+          filters: *only_main_branch_filter
+      - mdc_snapshot_test_cronjob:
+          filters: *only_main_branch_filter
 
     triggers:
       - schedule: