ci: bump actions/setup-node from 4 to 6

Bumps [actions/setup-node](https://github.com/actions/setup-node) from 4 to 6.
- [Release notes](https://github.com/actions/setup-node/releases)
- [Commits](actions/setup-node@v4...v6)

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

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

Author: dependabot[bot]

.github/workflows/blackroad-ci.yml

@@ -141,7 +141,7 @@ jobs:
       # Only run npm tests if package.json exists with a real test script
       - name: Setup Node.js
         if: hashFiles('package.json') != ''
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: 22
         continue-on-error: true