Remove 'pull_request_target' trigger event

As this event triggers build against of the base of the pull request,
the build triggered by it will fail when PR fixes already failing build
of the base.

.github/workflows/main.yml

@@ -1,5 +1,5 @@
 name: Sample workflow for App Center action
-on: [push, pull_request, workflow_dispatch, pull_request_target]
+on: [push, pull_request, workflow_dispatch]
 jobs:
   run:
     runs-on: ubuntu-latest