fix gha

Author: gmanchon

.github/workflows/build.yml

@@ -6,11 +6,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - name: Install dependencies
+        run: bundle install
       - name: Build
         run: |
           git config user.name github-actions
           git config user.email github-actions@github.com
-          ruby build.rb
+          bundle exec ruby build.rb
           if ! git diff --exit-code
           then
             git add .