Vendored Ruby tweaks

[MikeMcQuaid]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
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• fix original setup ruby path name.
• allow forcing the usage of the vendored Ruby.
• brew.sh: only setup/install Ruby when running Ruby.

CC @xu-cheng

[MikeMcQuaid]

Revealed this on first install:

mikebook # HOMEBREW_FORCE_VENDOR_RUBY=1 brew info                          (master) [/usr/local]
git: The build tool has reset ENV; --env=std required.
git: The build tool has reset ENV; --env=std required.
66 kegs, 44,693 files, 1.4G

[MikeMcQuaid]

@xu-cheng Without this if there's a bug in vendor-install we cannot get people to brew update to fix it.

[xu-cheng]

This is incorrect because ENV/scm/git which is used by update requiring Ruby path set.

[xu-cheng]

The long term solution is to rewrite scm script in bash.

[MikeMcQuaid]

Or we can avoid using ENV/scm/git in update again (which feels like a better solution for now)

[MikeMcQuaid]

As we want to avoid calling any Ruby code until we get to update-report (which should call vendor-install).

[xu-cheng]

Would you mind to wait for a moment? As I think porting to bash is not too difficult and much better solution.

[MikeMcQuaid]

Ok. Any idea how long?

[xu-cheng]

#496

[MikeMcQuaid]

@xu-cheng 👏

[xu-cheng]

I think you can inline this block into below if [[ -n "$HOMEBREW_BASH_COMMAND" ]] ... instead of adding an extra condition check.

[MikeMcQuaid]

@xu-cheng Are you 👍 on this now?

[xu-cheng]

One comment. Otherwise 👍