FIX: ensure plugin's gems are in the gem path (PR #12727)

Let’s say you want to use a gem in a plugin that has a dependencie.

You would write something like this:

gem 'dependency-gem', '1.2.3'
gem 'amazing-gem', '4.5.6'

However, since when we install a plugin gem we install it in the gems directory created inside the plugins directory, when it comes the time to install the amazing-gem, it won’t be able to find the dependency-gem.

This fixes that issue by adding the gems plugins directory to the global gem path.

Also fixed a frozen string error when specifying a source.

GitHub

feels like it is correct to me, maybe rubygems changed a bit over the years.