Whenever GOPATH is not defined in the environment, use $HOME/go
as its default value. For Windows systems use %USERPROFILE%/go
and $home/go for plan9.
The choice of these environment variables is based on what Docker
currently does. The os/user package is not used to avoid having
a cgo dependency.
Updates #17262. Documentation changes forthcoming.
TryBot-Result: Gobot Gobot <firstname.lastname@example.org>
Reviewed-by: Russ Cox <email@example.com>