only build on push to master

we have now merge queues for maintainance branches. We still build it
for master to have our installer beeing updated. In future this part
could go in new workflow instead.

.github/workflows/ci.yml

@@ -4,6 +4,8 @@ on:
   pull_request:
   merge_group:
   push:
+    branches:
+      - master
   workflow_dispatch:
     inputs:
       dogfood: