cmd/go: use builtin version and reexec for env

cmd/go: use builtin version and reexec for env

Skip execing for version (most reliable) and attempt to reexec
for env, falling back to the go in GOROOT

Fixes #43981

Change-Id: I17fb84d36036807274eecca3d4f64b3add9b9483
Reviewed-on: https://go-review.googlesource.com/c/go/+/288693
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>