With go 1.18, we must use go install for a binary (#1726)

The format "go install github.com/kyoh86/richgo@latest" will work with
go 1.16 and higher.  It will not work with go 1.15.  However, since
installing "richgo" is only required for people who want to run the go
tests for the Cobra project itself, I feel it is ok to require go 1.16
or higher in this case.

Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>

Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>