Run stepup-behat tests only once

Prior to this change, the stepup-behat tests would almost always run twice on every commit: one for 'push' and one for 'pull_request'.

This change only runs the 'push' run on the main branch.

This means, if there is no PR, it will also not run the tests on other branches.
`workflow_dispatch` adds the option to start the pipeline manually.

Running the tests twice caused issues with cdns (npm, composer) on other projects.

Author: johanib

.github/workflows/stepup-behat.yml

@@ -3,7 +3,9 @@ name: stepup-behat
 on:
   pull_request:
   push:
-    branches: [ main, feature/*, bugfix/* ]
+    branches:
+      - main
+  workflow_dispatch:
 jobs:
   build:
     runs-on: ubuntu-latest