Makefile: use go install for install-gen-deps
Makefile: use go install for install-gen-deps

The `go get' command for installing a module without modifying the
go.mod/go.sum was deprecated for several versions, and is now removed in
go 1.18.

Since the project supports go 1.17 at minimum, the behaviour is
supported too, and we can change this in the Makefile so the build works
on Go 1.18 too.

hashicorp/packer,hashicorp/packer,hashicorp/packer,hashicorp/packer