Add missing checkout for pages build

.github/workflows/ci-pipeline.yml

@@ -458,6 +458,40 @@ jobs:
         dotnet-version: '${{ env.TGS_DOTNET_VERSION }}.0.x'
         dotnet-quality: ${{ env.TGS_DOTNET_QUALITY }}
 
+    - name: Checkout (Branch)
+      uses: actions/checkout@v4
+      if: github.event_name == 'push' || github.event_name == 'schedule'
+
+    - name: Checkout (PR Merge)
+      uses: actions/checkout@v4
+      if: github.event_name != 'push' && github.event_name != 'schedule'
+      with:
+        ref: "refs/pull/${{ inputs.pull_request_number}}/merge"
+
+    - name: Read Current SHA
+      id: get-pr-sha
+      if: github.event_name != 'push' && github.event_name != 'schedule'
+      shell: bash
+      run: echo "head_sha=$(git rev-parse HEAD)" >> $GITHUB_OUTPUT
+
+    - name: Abort if PR Merge SHA has Changed
+      uses: actions/github-script@v7
+      if: github.event_name != 'push' && github.event_name != 'schedule' && steps.get-pr-sha.outputs.head_sha != github.sha
+      with:
+        script: |
+          const delay = ms => new Promise(res => setTimeout(res, ms));
+
+          github.rest.actions.cancelWorkflowRun({
+            owner: context.repo.owner,
+            repo: context.repo.repo,
+            run_id: context.runId
+          });
+
+          while (true) {
+            core.info('Waiting for workflow to cancel ...');
+            await delay(5000);
+          }
+
     - name: gh-pages Clone
       run: git clone -b gh-pages --single-branch "https://git@github.com/tgstation/tgstation-server" $HOME/tgsdox