Bump actions/download-artifact from 4 to 6

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](https://github.com/actions/download-artifact/compare/v4...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>

.github/workflows/build-linux.yml

@@ -116,7 +116,7 @@ jobs:
         fetch-depth: '0'
 
     - name: Restore build artifacts
-      uses: actions/download-artifact@v4
+      uses: actions/download-artifact@v6
       with:
         name: v2rayN-linux
         path: ${{ github.workspace }}/v2rayN/Release