Allow generating when BUNDLE_PATH is set

[jcoyne]

Fixes #3284

[jrochkind]

I wonder what the heck that with_unbundled_env was for in the first place! Hopefully nothing it was an unneeded accidental addition?

[jrochkind]

For code archeology... the code (in form of older method with_clean_env seems to have been added here:

b3831f8

With commit message just saying "Update install generators to explicitly run bundle install inside the destination_root". I don't totally understand what that means in this context.

By @cbeer . Should we check with @cbeer to see if he remembers the motivation or any reason this revert would break anything?

[cbeer]

with_unbundled_env was for in the first place!

I think the failing CI build demonstrates the kind of failure you get without it. If the CI build can be fixed, great 🤷‍♂️