ci: trigger build for pull requests

Author: zehnm

.github/workflows/build.yml

@@ -7,6 +7,9 @@ on:
     branches: [main]
     tags:
       - v[0-9]+.[0-9]+.[0-9]+*
+  pull_request:
+    branches: [main]
+    types: [opened, synchronize, reopened]
 
 env:
   INTG_NAME: androidtv