ci: unitctl: Bump actions/download-artifact from 4 to 5

Bumps <https://github.com/actions/download-artifact> from 4 to 5.

Link: Release notes <https://github.com/actions/download-artifact/releases>
Link: Commits <actions/download-artifact@v4...v5>
Signed-off-by: dependabot[bot] <support@github.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Author: dependabot[bot]

.github/workflows/unitctl.yml

@@ -178,7 +178,7 @@ jobs:
     needs: [build]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@v5
         with:
           merge-multiple: true