Mk/Uses/go.mk: fix gomod-clean target when lang/go is not installed

make distclean target will break when lang/go is not installed. This
provides an informational feedback and skips the gomod-clean target if
go executable is not available.

PR: 258359
MFH: 2021Q4
Submitted by: Rin Morningstar <ports.maintainer@evilphi.com>
Reviewed by: dmgk
Differential Revision: https://reviews.freebsd.org/D32054