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>

Makefile

@@ -6,7 +6,7 @@ $(warning "could not find golangci-lint in $(PATH), run: curl -sfL https://insta
 endif
 
 ifeq (, $(shell which richgo))
-$(warning "could not find richgo in $(PATH), run: go get github.com/kyoh86/richgo")
+$(warning "could not find richgo in $(PATH), run: go install github.com/kyoh86/richgo@latest")
 endif
 
 .PHONY: fmt lint test install_deps clean