Generate buildroot image tag dynamically

The buildroot tag should always be associated to the date it was created
rather than the date the review is submitted.

.github/workflows/buildroot.yml

@@ -15,11 +15,13 @@ jobs:
 
     env:
       IMAGE_REPO: ghcr.io/${{ github.repository_owner }}/shim-review-buildroot
-      IMAGE_TAG: endless-shim-x64-20240822
 
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+      - name: Generate image tag name
+        run: |
+          echo "IMAGE_TAG=endless-shim-x64-$(date -u +%Y%m%d)" >> "$GITHUB_ENV"
       - name: Build image
         run: docker build -f Dockerfile-buildroot -t "$IMAGE_REPO:$IMAGE_TAG" .
       - name: Registry login