ci: Checkout latest merged pulls

Github-Pull: #33303
Rebased-From: fa8f081af3

.github/workflows/ci.yml

@@ -108,8 +108,12 @@ jobs:
       BASE_ROOT_DIR: ${{ github.workspace }}
 
     steps:
-      - name: Checkout
-        uses: actions/checkout@v4
+      - &CHECKOUT
+        name: Checkout
+        uses: actions/checkout@v5
+        with:
+          # Ensure the latest merged pull request state is used, even on re-runs.
+          ref: &CHECKOUT_REF_TMPL ${{ github.event_name == 'pull_request' && github.ref || '' }}
 
       - name: Clang version
         run: |
@@ -165,8 +169,7 @@ jobs:
       TEST_RUNNER_TIMEOUT_FACTOR: 40
 
     steps:
-      - name: Checkout
-        uses: actions/checkout@v4
+      - *CHECKOUT
 
       - name: Configure Developer Command Prompt for Microsoft Visual C++
         # Using microsoft/setup-msbuild is not enough.
@@ -419,8 +422,7 @@ jobs:
             file-env: './ci/test/00_setup_env_native_msan.sh'
 
     steps:
-      - name: Checkout
-        uses: actions/checkout@v5
+      - *CHECKOUT
 
       - name: Configure environment
         uses: ./.github/actions/configure-environment
@@ -463,6 +465,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v5
         with:
+          ref: *CHECKOUT_REF_TMPL
           fetch-depth: 0
 
       - name: Configure Docker