homebrew install dir incorrect for apple silicon based macs

[tarakc02]

By default, on M1-based macs the homebrew install dir is /opt/homebrew, instead of /usr/local as expected by bin/libsnap.sh. This commit got things working for me on my new machine, but haven't checked if it works in general on other versions of homebrew. as a workaround, the user can always symlink the required executables into /usr/local as the error message describes, but if brew --prefix works in general that would be preferable. will check on other machines.