Set 'BUNDLE_LOCKFILE' to user's lock file

[g-arjones]

This is also required on Bundler 4.0.4+

[g-arjones]

@doudou Implemented this on top of your bundler-4 branch, but feel free to change the base if you want.

[doudou]

The default lockfile is not GEMFILE.lock anymore ?

[g-arjones]

It is, but require 'bundler/setup' now sets BUNDLE_LOCKFILE explicitly in the shims. Then, when BundlerManager initializes the environment and sets BUNDLE_GEMFILE to prefix/gems/Gemfile, we end up with one pointing to the user Gemfile and the other to the internal lock file.

To make matters even worse, if we only set BUNDLE_LOCKFILE in BundlerManager#initialize_environment and not in the shims, when you source env.sh you will have BUNDLE_LOCKFILE pointing to the user's lock file and BUNDLE_GEMFILE pointing to the internal gem file (set in the shim) which will cause bundler to refuse to load completely due to missing gems in the lock file.