Bump actions/download-artifact from 3 to 4

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 3 to 4.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v3...v4)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

action.yaml

@@ -66,7 +66,7 @@ runs:
         sudo apt-get install -y debos
     - name: Fetch prebuilt rootfs
       if: inputs.prebuilt-rootfs == 'true'
-      uses: actions/download-artifact@v3
+      uses: actions/download-artifact@v4
       with:
         name: rootfs_${{ env.suite }}_${{ inputs.flavor }}
         path: .rootfs