Tell the Action where to put the gems

This needs to match our custom setup in the Rakefile

Author: PeterJCLaw

.github/workflows/validate.yml

@@ -11,6 +11,8 @@ on:
 jobs:
   validate:
     runs-on: ubuntu-latest
+    env:
+      BUNDLE_PATH: gems
     steps:
     - uses: actions/checkout@v2
 
@@ -19,6 +21,7 @@ jobs:
       with:
         ruby-version: 2.7
         bundler-cache: true
+        cache-version: 1
 
     - name: Build
       run: bundle exec rake build