fix(github/ci): Checkout correct repo and branch on PR open

It seems like that since the introduction of CI Checks for Pull Requests, the Action run has always checked out the upstreams main branch instead of the PR branch of the fork. This fixes that.

.github/workflows/ci_build.yml

@@ -40,8 +40,8 @@ jobs:
           # Call each git command individually, preventing unnecessary checkouts
           git init .
           git config core.symlinks true
-          git remote add origin https://github.com/$GITHUB_REPOSITORY.git
-          git pull origin $GITHUB_REF:PR_BRANCH --depth=1
+          git remote add origin ${{ github.event.pull_request.head.repo.clone_url }}
+          git pull origin ${{github.event.pull_request.head.ref}} --depth=1
           git submodule update --jobs=$JOB_SLOTS --init --depth=1
         
       - name: Dependencies