Okay, so I think this is what happened to me:
1) I had brew installed on my Intel Mac
2) I got an Apple Silicon Mac
3) I setup my new Mac by copying over Intel Mac harddrive
4) I now have two brew's installed on my machine:
a) /usr/local/bin/brew (for intel macs & cross-compiling)
b) /opt/homebrew/bin/brew (for (for apple silicon)
5) The wrong brew was in my path (a)
Looking through our getting started docs, we seem to reference both /usr/local/... and opt/homebrew/...
Update the installation document to mac this and related issues more clear for new users who might run into this problem.
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