Bump actions/download-artifact from 4 to 7 (#5465)

Bumps
[actions/download-artifact](https://github.com/actions/download-artifact) from 4 to 7.

- vLLM version: v0.13.0
- vLLM main:
5326c89803

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

.github/workflows/_pr_image_build.yaml

@@ -103,14 +103,14 @@ jobs:
         uses: actions/checkout@v6
 
       - name: Download arm64 digests
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v7
         with:
           path: ${{ runner.temp }}/digests
           pattern: digests-${{ inputs.suffix }}-arm64
           merge-multiple: true
 
       - name: Download amd64 digests
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v7
         with:
           path: ${{ runner.temp }}/digests
           pattern: digests-${{ inputs.suffix }}-amd64