ci: remove Gemfile.lock before set up Ruby in test job

Author: nicolastemciuc

.github/workflows/build.yml

@@ -74,6 +74,7 @@ jobs:
       BUNDLE_GEMFILE: gemfiles/${{ matrix.gemfile }}.gemfile
     steps:
     - uses: actions/checkout@v2
+    - run: rm Gemfile.lock
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}