Fix documentation for GoCmdEnv

[ihgann]

GoCmdEnv documentation was a copy of the verbose mode documentation - this change specifies the intention of the -gocmd flag.

[ihgann]

I had a further question that might be better tracked as an issue - I use my magefile with some specific targets that utilize the go runtime, and in particular it's a bit confusing that setting -gocmd does not actually persist when I invoke mg.GoCmd() within my own magefile.

That is to say, -gocmd specifically only applies to compiling the magefile, but this seems misleading. I think if -gocmd is applied, then the magefile ought to be run with var MAGEFILE_GOCMD set as well. That doesn't apply to this change per se, but I only noticed this documentation note because I was confused by this behaviour.

[natefinch]

Thanks!

[natefinch]

That's a good point about it not getting passed through. Definitely worth an issue and something that is easy to fix.

[ihgann]

@natefinch Cool - maybe I can take a stab at it :)