Rationale: some users insist on replacing the /usr/bin/ruby symlink
to point to another ruby on their system, which may break homebrew.
Use the full Framework path instead, which is less likely to be tampered with.

This also reorganizes the brew --config checks to reflect the different path.

Fixes Homebrew/homebrew#12009.

Closes Homebrew/homebrew#12333.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>