[Github] Make litmus-tests workflow only run for push events on main (#504)

Without this change the workflow will run twice on stacked PRs as there
is a push event to the repo and a pull_request event. Not a big deal,
but a slight annoyance that is an easy fix.

Author: boomanaiden154

.github/workflows/litmus-tests.yml

@@ -1,6 +1,10 @@
 name: llvm-zorg testing
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
 
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   contents: read