deps: do not install go if it's available on the system

[rjeczalik]

Sometimes it may happen that user chose to install go either:

+ by hand by unzipping the Go distibution
+ by Mac installer
= not using homebew

Having [0] would make homebrew to install Go for the second time, which may cause sometimes a lot of subtle problems, since type go will point to homebrew's Go but GOROOT to the old, system one.

I think what would be helpful is to peek if go exist first, and then install it if needed.

[0] https://github.com/go-delve/homebrew-delve/blob/master/Formula/delve.rb#L10