Limit codeQL push trigger to main branch (#1507)

I noticed the codeQL action running twice on a PR I updated, once
because of the pull_request trigger, and once because of the push
trigger.

Signed-off-by: Spencer Schrock <sschrock@google.com>
Co-authored-by: Stephen Augustus <justaugustus@users.noreply.github.com>

.github/workflows/codeql-analysis.yml

@@ -13,6 +13,7 @@ name: "CodeQL"
 
 on:
   push:
+    branches: [ main ]
   pull_request:
     # The branches below must be a subset of the branches above
     branches: [ main ]