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: d8293c4729/lib/rubygems/commands/install_command.rb (L151)
Blame: 9ea600c9c2
This is a hack that sets the :install_dir to where we would install
anyways (the GEM_HOME is the default installation destination).
bundler for example needs to have the GEM_HOME being passed trough to
function properly.
For gems that are loading content dynamically, or can use plugins, use
buildRubyGem.
For executables that are wrapped in their own sealed thing use
bundlerEnv.
lib.escapeShellArgs doesn't work well when a null value is provided.
[] is also the correct value since it's really just an empty list of
arguments that we have.
