Update CI to support running CI against forks

Using `pull_request_target` instead of `pull_request` is supposed
to enable this...

Author: aron

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ name: CI
 on:
   push:
     branches: ["main"]
-  pull_request:
+  pull_request_target:
     branches: ["main"]
 
 jobs: