[ci] enable parallel jobs for ruby workflow

Github Actions allow to run jobs in parallel which can significantly
speed up the CI process.

Signed-off-by: Jean-Noël Avila <[email protected]>

Author: jnavila

.github/workflows/ruby.yml

@@ -34,4 +34,4 @@ jobs:
     - name: Install dependencies
       run: bundle install
     - name: Run tests
-      run: bundle exec make all
+      run: bundle exec make -j8 all