Bump actions/download-artifact from 7 to 8 (#6907)

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 7 to 8.

- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/_schedule_image_build.yaml

@@ -115,14 +115,14 @@ jobs:
         uses: actions/checkout@v6
 
       - name: Download arm64 digests
-        uses: actions/download-artifact@v7
+        uses: actions/download-artifact@v8
         with:
           path: ${{ runner.temp }}/digests
           pattern: digests-${{ inputs.suffix }}-arm64
           merge-multiple: true
 
       - name: Download amd64 digests
-        uses: actions/download-artifact@v7
+        uses: actions/download-artifact@v8
         with:
           path: ${{ runner.temp }}/digests
           pattern: digests-${{ inputs.suffix }}-amd64