bundler/inline should always install gems to system path
bundler/inline should always install gems to system path

As discussed in the P/R, when `BUNDLE_PATH` env is set Bundler should
still install gems to the system path. `GEM_HOME` can be used to provide
different location if needed.

The test is added to document expected behavior of `bundler/inline`.

mvz/bundler,mvz/bundler,bundler/bundler,bundler/bundler