[rubygems/rubygems] Fix unintended Gemfile installation

This command is failing because of the same reason that the subsequent
`bundle exec` is failing: the gemspec is invalid. The `bundle install`
here deviates the `bundle exec` focus from the test and is unnecessary:
all we need is a `Gemfile` that will trigger the `bundle exec`, so let's
create and avoid the extra command.

https://github.com/rubygems/rubygems/commit/eb83cf6cf1

spec/bundler/commands/exec_spec.rb

@@ -576,7 +576,7 @@ RSpec.describe "bundle exec" do
         G
       end
 
-      install_gemfile <<-G
+      gemfile <<-G
         gem "foo", :path => "#{lib_path("foo-1.0")}"
       G