Mk/Uses/go.mk: Allow ports to have additional distfiles

Some ports need to perform extra build steps for which they need additional distfiles.

This commit makes go.mk to preserve MASTER_SITES,DISTFILES,EXTRACT_ONLY set by port's Makefile.

PR:		255085
Approved by:	dmgk (maintainer)

Mk/Uses/go.mk

@@ -124,9 +124,9 @@ GO_MODVERSION=	${GO_MODULE:C/^([^@]*)(@([^@]*)?)/\2/:M@*:S/^@//:S/^$/${DISTVERSI
 GO_MODFILE=	${GO_MODVERSION}.mod
 GO_DISTFILE=	${GO_MODVERSION}.zip
 DIST_SUBDIR=	go/${PKGORIGIN:S,/,_,g}/${DISTNAME}
-MASTER_SITES=	${GO_GOPROXY}/${GO_MODNAME:C/([A-Z])/!\1/g:tl}/@v/
-DISTFILES=	${GO_MODFILE} ${GO_DISTFILE}
-EXTRACT_ONLY=	${GO_DISTFILE}
+MASTER_SITES+=	${GO_GOPROXY}/${GO_MODNAME:C/([A-Z])/!\1/g:tl}/@v/
+DISTFILES+=	${GO_MODFILE} ${GO_DISTFILE}
+EXTRACT_ONLY+=	${GO_DISTFILE}
 WRKSRC=		${WRKDIR}/${GO_MODNAME}@${GO_MODVERSION}
 FETCH_DEPENDS+=	${GO_CMD}:${GO_PORT} \
 		ca_root_nss>0:security/ca_root_nss