Change CI workflow branch from 'main' to 'master'

to d2dyno: i didn't open a pr because this was a small and silly error (oversight)

Author: Lamparter

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ name: CI
 on:
   push:
     branches:
-      - main
+      - master
   pull_request:
   workflow_dispatch: