Set GEM_HOME including current ABI Ruby version

[vifreefly]

Hello! Thanks for the snap Ruby package, it works pretty well. I have one question though:

By default snap Ruby package install gems into ~/.gem directory, which is of course better than installing system-wide (don't need sudo). But I found this behavior a little bit confusing, because it didn't match --user-install behavior where everything goes into ~/.gem/ruby/current_ABI_ruby_version_number rather than just to ~/.gem.

Is current behavior is expected?