chore: Reduce E2E test jobs run on PRs

The number of E2E test jobs run on PRs has been reduced to save on
CircleCI credits. We still run the "chrome MV3" test job, but the
Firefox and "chrome MV2/webpack build" E2E test jobs are now only run
on `develop`, `master`, and RC branches. This should result in huge
CircleCI credit savings.

These jobs were chosen because it's uncommon for test failures or
flakiness to manifest in these jobs without also appearing in the
Chrome MV3 E2E test job, and this job represents the mmajority of our
userbase (the Chrome MV2/webpack build is only used for development).

.circleci/config.yml

@@ -197,6 +197,7 @@ workflows:
           requires:
             - prep-deps
       - test-e2e-chrome-webpack:
+          <<: *develop_master_rc_only
           requires:
             - prep-build-test-webpack
             - get-changed-files-with-git-diff
@@ -205,6 +206,7 @@ workflows:
             - prep-build-test
             - get-changed-files-with-git-diff
       - test-e2e-firefox:
+          <<: *develop_master_rc_only
           requires:
             - prep-build-test-mv2
             - get-changed-files-with-git-diff