Suggest absolute path for BUNDLE_GEMFILE

This ensures the gemfile can be found when a step changes the working
directory.

Author: jonathanhefner

README.md

@@ -108,7 +108,7 @@ jobs:
         gemfile: [ rails5, rails6 ]
     runs-on: ubuntu-latest
     env: # $BUNDLE_GEMFILE must be set at the job level, so it is set for all steps
-      BUNDLE_GEMFILE: gemfiles/${{ matrix.gemfile }}.gemfile
+      BUNDLE_GEMFILE: ${{ github.workspace }}/gemfiles/${{ matrix.gemfile }}.gemfile
     steps:
       - uses: actions/checkout@v2
       - uses: ruby/setup-ruby@v1