ci: avoid installing node modules again

By default `bin/rails assets:precompile` will run `yarn install` again.

The suggested workaround is to remove `bin/yarn` before running the
task.

See https://github.com/rails/webpacker/issues/405

.github/workflows/ci.yml

@@ -107,6 +107,7 @@ jobs:
         env:
           RAILS_ENV: test
         run: |
+          rm bin/yarn
           bin/rails assets:precompile --trace
 
       - name: Run tests