Revert "github actions: change github.event.pull_request event"

This reverts commit 27c6f41.

Author: PlaidCat

.github/workflows/push-check_x86_64.yml

@@ -24,7 +24,7 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
         with:
-          ref: "${{ github.event.pull_request.head }}"
+          ref: "${{ github.event.pull_request.merge_commit_sha }}"
           fetch-depth: 0
       - name: Build the Kernel
         run: |