Fix brew prof --stackprof

The `stackprof` gem wasn't being included
correctly because it was missing the path
directory. This vendored gems path is added
later on in `load_path.rb`.

We export the GEM_PATH and GEM_HOME environment
variables in setup_gem_environment! so we
can use that information if we require the
`rubygems` gem first.

Library/Homebrew/brew.rb

@@ -2,6 +2,7 @@
 # frozen_string_literal: true
 
 if ENV["HOMEBREW_STACKPROF"]
+  require "rubygems"
   require "stackprof"
   StackProf.start(mode: :wall, raw: true)
 end