test: don't run browser workflow for pull_request event

Since github workflows triggered from pull requests from forked repos
cannot access the secrets (e.g. Browserstack credentials) it doesn't
make sense to run these checks on the pull request event.

.github/workflows/browser.yml

@@ -1,6 +1,6 @@
 name: Browser
 
-on: [push, pull_request]
+on: [push]
 
 jobs:
   ci: