CI: undo main -> $default-branch (cannot be used in workflows)

`$default-branch` is a macro only available in _workflow templates_,
not in actual workflows.

Author: andreasabel

.github/workflows/workflow.yml

@@ -2,12 +2,13 @@ name: "/setup"
 on:
   push:
     branches:
-      - $default-branch
+      - main
     paths-ignore:
       - "**.md"
   pull_request:
     paths-ignore:
       - "**.md"
+  workflow_dispatch:
 
 defaults:
   run: