gha: Rebase atop of the target branch

We have two scenarios we care about this, `pull_request` and
`pull_request_target` events triggered a job.

`pull_request` event:
When using the checkout action, it'll already provide a "rebased atop of
main" repo for us, nothing else is needed, and that's basically what we
already have as part of the jobs in our CI.

`pull_request_target` event:
This one is a little bit tricky, as the checkout action, unless passing
a spsecific repo, give us the PR checked out rebased atop of the HEAD of
the PR branch.  Jeremi Piotrowski nicely pointed out that we could use
github.event.pull_request.merge_commit_sha instead, which is the result
of the PR's branch with the official repo target branch.

Now, the only cases where the contributor's rebase would still be needed
is when the action itself has been changed.

Fixes: #7414

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

.github/workflows/add-pr-sizing-label.yaml

@@ -22,6 +22,8 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.merge_commit_sha }}
 
       - name: Install PR sizing label script
         run: |

.github/workflows/ci-on-push.yaml

@@ -25,7 +25,7 @@ jobs:
     if: ${{ contains(github.event.pull_request.labels.*.name, 'ok-to-test') }}
     uses: ./.github/workflows/ci.yaml
     with:
-      commit-hash: ${{ github.event.pull_request.head.sha }}
+      commit-hash: ${{ github.event.pull_request.merge_commit_sha }}
       pr-number: ${{ github.event.pull_request.number }}
       tag: ${{ github.event.pull_request.number }}-${{ github.event.pull_request.head.sha }}
     secrets: inherit

.github/workflows/move-issues-to-in-progress.yaml

@@ -39,6 +39,8 @@ jobs:
       - name: Checkout code to allow hub to communicate with the project
         if: ${{ !contains(github.event.pull_request.labels.*.name, 'force-skip-ci') }}
         uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.merge_commit_sha }}
 
       - name: Move issue to "In progress"
         if: ${{ !contains(github.event.pull_request.labels.*.name, 'force-skip-ci') }}

.github/workflows/require-pr-porting-labels.yaml

@@ -37,6 +37,8 @@ jobs:
       - name: Checkout code to allow hub to communicate with the project
         if: ${{ !contains(github.event.pull_request.labels.*.name, 'force-skip-ci') }}
         uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.merge_commit_sha }}
 
       - name: Install porting checker script
         run: |