buildRubyGem: handle build-time ruby dependencies

For some reason `gem install` unsets the GEM_PATH environment variable
internally unless the install dir is provided. This in turn means that
if it invokes extconf.rb and extconf.rb depends on a gem available on
the GEM_PATH (like pkg-config for nokogiri) then it's not available in
that context.

Proof: https://github.com/rubygems/rubygems/blob/d8293c472966cb1de08784dfdd90560e56ceed84/lib/rubygems/commands/install_command.rb#L151
Blame: https://github.com/rubygems/rubygems/commit/9ea600c9c2b77d1459526fe1e2bcdf2ac778c24a

This is a hack that sets the :install_dir to where we would install
anyways (the GEM_HOME is the default installation destination).