cmd/go: introduce GOROOT/go.env and move proxy/sumdb config there

cmd/go: introduce GOROOT/go.env and move proxy/sumdb config there

Various Linux distributions edit cmd/go/internal/cfg/cfg.go to change
the default settings of GOPROXY and GOSUMDB. Make it possible for
them to do this without editing the go command source code by
introducing GOROOT/go.env and moving those defaults there.

With the upcoming changes for reproducible builds (#24904),
this should mean that Linux distributions distribute binaries
that are bit-for-bit identical to the Go distribution binaries,
even when rebuilding the distribution themselves.

Fixes #57179.

Change-Id: Ib2ecc61e6d036f97db6fd47dca757c94fdea5629
Reviewed-on: https://go-review.googlesource.com/c/go/+/462198
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>