cmd/go: add GOEXPERIMENT to `go env` output

cmd/go: add GOEXPERIMENT to `go env` output

This CL adds GOEXPERIMENT to `go env` output, and also makes it
configurable via `GOENV`. Thanks to Baokun Lee's CL 304350 for the
test and initial work on this.

Fixes #45226.

Change-Id: Ie7f92a8a503b6a2a4df3f6598f0b2bf2915e2e7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/328751
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>