ci: disable trigger-snap for PRs

Fixes #8568

This will hide snap issues from PRs which is not ideal but we can
address that in the future. We still run snap CI for main.

.github/workflows/test.yml

@@ -273,6 +273,7 @@ jobs:
             ghostty-source.tar.gz
 
   trigger-snap:
+    if: github.event_name != 'pull_request'
     runs-on: namespace-profile-ghostty-xsm
     needs: build-dist
     steps: