brew.sh: enforce HOMEBREW_FORCE_BREW_WRAPPER more strictly

[carlocab]

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HOMEBREW_FORCE_BREW_WRAPPER can be used as a security/compliance
feature, but allowing it to be disabled by setting
HOMEBREW_NO_FORCE_BREW_WRAPPER leaves a pretty large hole in it that
allows it to be sidestepped.

Let's fix that by actually checking the path of the process that called
brew, and the verify that that path matches the configured value of
HOMEBREW_NO_FORCE_BREW_WRAPPER.