github actions: Make builds on Merge Request

Since we need to make sure all our checks run BEFORE the commit is
pushed to a branch we're swithcing this to `on pull_request_target` to
enable external contibutors.

Author: PlaidCat

.github/workflows/push-check_x86_64.yml

@@ -1,6 +1,6 @@
 name: CI
 on:
-  push:
+  pull_request_target:
     branches:
       - '**'
       - '!mainline'