Download gems that are not present, like net-imap dependencies

Some of those dependencies are part of Ruby already, so they aren't
installed locally. If we use local bundler will skip downloading them
but will not be able to find them anyway, failing.

test/support/acceptance_test.rb

@@ -664,7 +664,7 @@ def exec_name
 
         FileUtils.cp_r "#{app.gem_home}/", bundle_path.to_s
 
-        app.run! "bundle install --path .bundle --clean --local"
+        app.run! "bundle install --path .bundle --clean --prefer-local"
 
         assert_speedup do
           2.times { assert_success "bundle exec rails runner ''" }