Why isn't GOPRIVATE set?

gistfile1.txt

GO=go
GOBUILD=${GO} build
GOINSTALL=${GO} install
GOTEST=${GO} test
GOPROXY=https://proxy.golang.org,direct
GOPRIVATE=source.example.com
GOTOOL=${GO} tool

.PHONY: env
env:
        @echo "GOVERSION:   $(shell ${GO} version)"
        @echo "GITVERSION:  $(shell git --version)"
        @echo "GO111MODULE: $(shell ${GO} env GO111MODULE)"
        @echo "GOFLAGS:     $(shell ${GO} env GOFLAGS)"
        @echo "GOPRIVATE:   $(shell ${GO} env GOPRIVATE)"
        @echo "GOPROXY:     $(shell ${GO} env GOPROXY)"
        @echo "GONOPROXY:   $(shell ${GO} env GONOPROXY)"
        @echo "GOSUMDB:     $(shell ${GO} env GOSUMDB)"
        @echo "GONOSUMDB:   $(shell ${GO} env GONOSUMDB)"
        @echo "PATH:        $(PATH)"
        @echo "SHELL:       $(SHELL)"