[make] Use GOFLAGS instead of manually passing `-mod=vendor`

Makefile

@@ -3,12 +3,12 @@ SOURCE_DATE_EPOCH ?= $(shell date +%s)
 BUILD_DATE = $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" '+%d %b %Y' 2>/dev/null || date -u -r "$(SOURCE_DATE_EPOCH)" '+%d %b %Y')
 HUB_VERSION = $(shell bin/hub version | tail -1)
 FLAGS_ALL = $(shell go version | grep -q 'go1.[89]' || echo 'all=')
-export MOD_VENDOR_ARG := $(shell go version | grep -q 'go1.1[^01]' && echo '-mod=vendor')
+export GOFLAGS := $(shell go version | grep -q 'go1.1[^0]' && echo '-mod=vendor')
 export LDFLAGS := -extldflags '$(LDFLAGS)'
 export GCFLAGS := $(FLAGS_ALL)-trimpath '$(PWD)'
 export ASMFLAGS := $(FLAGS_ALL)-trimpath '$(PWD)'
 
-ifneq ($(MOD_VENDOR_ARG),)
+ifneq ($(GOFLAGS),)
 	export GO111MODULE=on
 	unexport GOPATH
 endif
@@ -52,7 +52,7 @@ bin/hub: $(SOURCES)
 	script/build -o $@
 
 bin/md2roff: $(SOURCES)
-	go build $(MOD_VENDOR_ARG) -o $@ github.com/github/hub/md2roff-bin
+	go build -o $@ github.com/github/hub/md2roff-bin
 
 test:
 	go test ./...

script/build

@@ -14,7 +14,6 @@ find_source_files() {
 build_hub() {
   mkdir -p "$(dirname "$1")"
   go build \
-	  $MOD_VENDOR_ARG \
 	  -ldflags "-X github.com/github/hub/version.Version=`./script/version` $LDFLAGS" \
 	  -gcflags "$GCFLAGS" \
 	  -asmflags "$ASMFLAGS" \