Mk/Uses/go.mk: Do not append to EXTRACT_ONLY

Appending to EXTRACT_ONLY may re-add files that were already excluded by
the port. Instead, set EXTRACT_ONLY only if it was not already defined
and let the port handle it otherwise.

Mk/Uses/go.mk

@@ -143,7 +143,7 @@ MASTER_SITES+=	${GO_GOPROXY}/${GO_MODNAME:C/([A-Z])/!\1/g:tl}/@v/
 DISTFILES+=	${GO_MODFILE} ${GO_DISTFILE}
 WRKSRC=		${WRKDIR}/${GO_MODNAME}@${GO_MODVERSION}
 .      endif
-EXTRACT_ONLY+=	${DISTFILES:N*.mod\:*:N*.mod:C/:.*//}
+EXTRACT_ONLY?=	${DISTFILES:N*.mod\:*:N*.mod:C/:.*//}
 DIST_SUBDIR=	go/${PKGORIGIN:S,/,_,g}/${DISTNAME}
 FETCH_DEPENDS+=	${GO_CMD}:${GO_PORT} \
 		ca_root_nss>0:security/ca_root_nss