ci: restrict push trigger to main branch

Avoid duplicate CI runs on PRs by limiting the push trigger to the
main branch only. PRs are already covered by the pull_request trigger.

Signed-off-by: jimisola <jimisola@jimisola.com>

Author: Jimisola Laursen

.github/workflows/build.yml

@@ -2,6 +2,8 @@ on:
   workflow_call:
   workflow_dispatch:
   push:
+    branches:
+      - main
   pull_request:
     types:
       - opened