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] <support@github.com>

.github/workflows/test-on-downstream-projects.yaml

@@ -36,7 +36,7 @@ jobs:
     steps:
       - run: git clone "https://gitlab.com/mailman/mailman/" .
       - name: download sdist build
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: latest-build
           path: nose2-build