cmd/go: error for unset $GOPATH when GOROOT is $HOME/go is unhelpful

[kortschak]

What version of Go are you using (go version)?

$ go version
go version go1.13.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/home/user/bin"
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/user/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/user/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build291059786=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Unset GOPATH and have the go source tree located at $HOME/go.
Then run the go tool to do something, e.g. go env.

What did you expect to see?

An informative error message.

What did you see instead?

"missing $GOPATH"

Additional information

See https://groups.google.com/d/topic/golang-nuts/D7wk6Mb6bZI/discussion

Note that while this is superficially similar to #29341, the fix at https://golang.org/cl/158257 will not fix this.

[andybons]

@bcmills @jayconrod