github actions: Fix upstream commit check for forks

The upstream commit check workflow was failing for pull requests
originating from forked repositories. The previous implementation
incorrectly assumed the pull request branch existed on the base repository.

This commit corrects the workflow to ensure the pull request branch
is checked out from the correct source repository, while the base
branch is fetched from the target repository.

Author: bmastbergen

.github/workflows/upstream-commit-check.yml

@@ -16,12 +16,14 @@ jobs:
       - name: Checkout PR branch
         uses: actions/checkout@v4
         with:
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
           fetch-depth: 0
           ref: ${{ github.head_ref }}
 
       - name: Checkout base branch
         run: |
-          git fetch origin ${{ github.base_ref }}:${{ github.base_ref }}
+          git remote add base_repo https://github.com/${{ github.repository }}.git
+          git fetch base_repo ${{ github.base_ref }}:${{ github.base_ref }}
 
       - name: Download check_kernel_commits.py
         run: |