Bump actions/download-artifact from 5 to 6 in /sign-pkg-mac

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

sign-pkg-mac/action.yaml

@@ -45,7 +45,7 @@ runs:
   using: composite
   steps:
     - name: Fetch Mac app
-      uses: actions/download-artifact@v5
+      uses: actions/download-artifact@v6
       with:
         name: macOS-app
         path: .tarball