Ensure go/bin is accessible or provide instructions on Linux

[flaki]

People trying to install the subo CLI seem to be hung up here, I just tried the clean install on Ubuntu 20.04 from main.

I got the latest go snap (sudo snap install go --classic), cloned the repo and make subo completes successfully, the ~/go/bin/subo executable is create but subo is not in the path. We either need to put more emphasis on getting go/bin on the path in the README or automate this as part of the Makefile.