bin/brew: re-exec self under Rosetta if needed

[mistydemeo]

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As discussed in #9177, we've reserved /usr/local as the prefix to use on Apple Silicon Macs for an Intel installation of Homebrew to be run under Rosetta.

If a user has a /usr/local installation on an Apple Silicon Mac, and executes /usr/local/bin/brew, we should make sure we're running that copy of brew as Intel under emulation in order to avoid confusion. This makes that change.

cc #9126

cc @claui, @MikeMcQuaid

[BrewTestBot]

Review period will end on 2020-11-26 at 01:48:37 UTC.

[MikeMcQuaid]

A few concerns:

• Will this succeed if Rosetta isn't installed?
• This will avoid the existing prefix messaging from ever being displayed for /usr/local which is undesirable.
• If the existing /usr/local had some ARM software installed in it this will produce confusing errors later on.
• I'd rather we avoided hardcoding /usr/local in another place here.
• It feels like there's a bit of overlap here with Check flavour to prevent mixed architectures #9126.

An alternate approach I'd rather (which overlaps with #9126) is changing or tweaking the existing error messages to make it more clear what needs to be done and how to do it. Doing so automatically without prompt or notification seems dangerous. Perhaps having an environment variable which has this behaviour (in brew.sh not bin/brew) would be an acceptable workaround after #9126 is finished and merged (or closed)

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Closing in favour of #9418 ❤️