FIX: Make bundler CI cache setup explicit (#10785)

Instructions on how to set this up exist at:

https://github.com/actions/cache/blob/master/examples.md#ruby---bundler

The second step was omitted, but it did not seem to prevent caching for unknown reasons.

.github/workflows/ci.yml

@@ -91,6 +91,7 @@ jobs:
           gem install bundler -v 2.1.4 --no-doc
           bundle config deployment 'true'
           bundle config without 'development'
+          bundle config path vendor/bundle
 
       - name: Bundler cache
         uses: actions/cache@v1