Fix issue preventing PR container builds

Checking out the PR fork repo could sometimes cause the "git diff" command to return an invalid revision range, like in [1].
This might happen if the fork repo is outdated.

[1] https://github.com/redhat-developer/rhdh-operator/actions/runs/12417737622/job/34715972414?pr=582

.github/workflows/pr-bundle-diff-checks.yaml

@@ -39,8 +39,7 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
         with:
           fetch-depth: 0
-          repository: ${{github.event.pull_request.head.repo.full_name}}
-          ref: ${{ github.event.pull_request.head.ref }}
+          ref: "${{ github.event.pull_request.merge_commit_sha }}"
 
       - name: Setup Go
         uses: actions/setup-go@3041bf56c941b39c61721a86cd11f3bb1338122a # v5

.github/workflows/pr-container-build.yaml

@@ -49,8 +49,7 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
         with:
           fetch-depth: 0
-          ref: ${{ github.event.pull_request.head.ref }}
-          repository: ${{ github.event.pull_request.head.repo.full_name }}
+          ref: "${{ github.event.pull_request.merge_commit_sha }}"
 
       # check changes in this commit for regex include and exclude matches; pipe to an env var
       - name: Check for changes to build