Pointing `Gem.ruby` to the newly installed ruby gives mkmf the right
inputs to build extensions in bundled gems. Previously, this patching
was only done for compressed bundled gems.
This patch also prevents `tool/fake.rb` from propagating to the child
process running mkmf for the native extension. The way `tool/fake.rb`
changes mkmf variables using `Kernel#trace_var` created spooky action at
a distance which made debugging difficult.
AppVeyor Windows CI started to fail starting with
8a3663789c because it enabled extension
building for bundled gems on mswin. This patch should address the CI
failures.