Avoid running all CI steps twice (#433)

Separate `push` and `pull_request` triggers so that pushing to a PR
branch doesn't trigger everything twice.

Author: ryanslade

.github/workflows/build.yml

@@ -1,5 +1,9 @@
 name: Build
-on: [push, pull_request]
+on:
+  pull_request:
+  push:
+    branches:
+      - main
 permissions:
   contents: write
   packages: write