Build scheduling changes

Run builds on pushes/merges to 'main', on PRs, and upon request.

.github/workflows/buildAndTest.yml

@@ -1,6 +1,12 @@
 name: Build and Test
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    types: [assigned, opened, synchronize, reopened]
+  workflow_dispatch:
 
 jobs:
   # Build the LLVM submodule then cache it. Do not rebuild if hit in the