Unify CI triggers across all gap-actions

Author: fingolfin

.github/workflows/CI.yml

@@ -1,15 +1,12 @@
 name: CI
 
-# Trigger the workflow on push or pull request
+# Trigger the workflow on push to main, or on pull request
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
 
-env:
-  CHERE_INVOKING: 1
-
 jobs:
   test:
     runs-on: ubuntu-latest