cmd/go: go env GOTOOLCHAIN prints superfluous trailing newline

[zephyr]

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

$ go version
go version devel go1.21-e4e8f9b8ff 2023-05-23 04:38:43 +0000 linux/amd64

Does this issue reproduce with the latest release?

No, because GOTOOLCHAIN is new in go1.21.

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

go env Output

$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN='/home/dennis/bin/binaries-golang'
GOCACHE='/home/dennis/.cache/go-build'
GOENV='/home/dennis/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/dennis/.local/share/gopath/pkg/mod'
GONOPROXY='bitbucket.org/zephyr'
GONOSUMDB='bitbucket.org/zephyr'
GOOS='linux'
GOPATH='/home/dennis/.local/share/gopath'
GOPRIVATE='bitbucket.org/zephyr'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/golang/master'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLDIR='/opt/golang/master/pkg/tool/linux_amd64'
GOVCS='*:git'
GOVERSION='devel go1.21-e4e8f9b8ff 2023-05-23 04:38:43 +0000'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3637338098=/tmp/go-build -gno-record-gcc-switches'
GOROOT/bin/go version: go version devel go1.21-e4e8f9b8ff 2023-05-23 04:38:43 +0000 linux/amd64
GOROOT/bin/go tool compile -V: compile version devel go1.21-e4e8f9b8ff 2023-05-23 04:38:43 +0000
uname -sr: Linux 5.10.0-22-amd64
Distributor ID:	Debian
Description:	Debian GNU/Linux 11 (bullseye)
Release:	11
Codename:	bullseye
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Debian GLIBC 2.31-13+deb11u6) stable release version 2.31.

What did you do?

After reading #57001:

$ go env -w GOTOOLCHAIN=local
$ go env GOTOOLCHAIN
$ go env --json GOTOOLCHAIN

What did you expect to see?

local
{
	"GOTOOLCHAIN": "local"
}

What did you see instead?

local

{
	"GOTOOLCHAIN": "local\n"
}

[ianlancetaylor]

CC @bcmills @rsc

[gopherbot]

Change https://go.dev/cl/497437 mentions this issue: cmd/go: set default GOTOOLCHAIN in GOROOT/go.env