ci: only trigger on push to master to avoid duplicate PR runs

These workflows triggered on both push and pull_request with no branch
filter, so every push to a PR branch ran each workflow twice. Restrict the
push trigger to master; pull_request already covers PR branches.

.github/workflows/linux.yml

@@ -1,6 +1,9 @@
 name: linux
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [master]
+  pull_request:
 
 concurrency:
   group: ${{ github.workflow }}-linux-${{ github.event.pull_request.number || github.ref }}