ci: harden label based browser runner

Author: voxpelli

.github/workflows/browser-test.yml

@@ -1,9 +1,14 @@
 name: Browser Tests for forked PRs
-'on':
-  pull_request_target:
+
+on:
+  pull_request:
     types:
       - labeled
 
+permissions:
+  contents: read
+  pull-requests: write
+
 jobs:
   test-browser:
     # TODO: configure to retain build artifacts in `.karma/` dir