Disable builds for branches that are not pull requests or main

Author: zanieb

.github/workflows/apple.yml

@@ -2,6 +2,7 @@ name: MacOS Python build
 
 on:
   push:
+    branches: [main]
   pull_request:
   schedule:
     - cron: '13 11 * * *'

.github/workflows/linux.yml

@@ -2,6 +2,7 @@ name: Linux Python build
 
 on:
   push:
+    branches: [main]
   pull_request:
   schedule:
     - cron: '13 11 * * *'

.github/workflows/windows.yml

@@ -2,6 +2,7 @@ name: Windows Python build
 
 on:
   push:
+    branches: [main]
   pull_request:
   schedule:
     - cron: '13 11 * * *'