ci: skip registry login for pull requests

PR builds never push, and Gitea gives them no secrets, so the login with
an empty token failed every PR build before it started.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -104,6 +104,8 @@ jobs:
           fetch-depth: 0
 
       - name: Log in to registry
+        # Only the push step needs it, and PR builds get no secrets.
+        if: github.event_name != 'pull_request'
         env:
           REGISTRY_USERNAME: ${{ secrets.REGISTRY_USERNAME }}
           REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN }}