Run CI on pull requests

If the PR is from an external repo,
the action won't run without approval.
right now we can't run the actions on external PRs.

.github/workflows/build.yml

@@ -1,4 +1,7 @@
-on: [ push ]
+on:
+  pull_request:
+  push:
+  workflow_dispatch:
 
 jobs:
   build_ubuntu:

.github/workflows/validate.yml

@@ -1,4 +1,5 @@
 on:
+  pull_request:
   push:
   workflow_dispatch: