[misc] Run CI on pushes to release branches

Extends every push-to-main CI workflow to also trigger on release-*
branches, so a backport merged to a release branch gets the same
post-merge test coverage main gets. Pull request triggers are already
branch-unfiltered, so backport PRs were covered; the gap was the branch
state after merging.

sync-main is deliberately untouched: it syncs main into a downstream
repository and must never run for a release branch.

.github/workflows/install-script-test.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - main
+      - "release-*"
   pull_request:
     paths:
       - "release_files/install.sh"