cmd/go: document GOCACHEPROG in go help environment
This adds GOCACHEPROG to the list of environment variables in "go help
environment" and points to the cacheprog package documentation for
details of the protocol.
Fixes #71032
Updates #59719
Change-Id: Ib8f5804926a8fa59237661076d129c2852665ac3
Reviewed-on: https://go-review.googlesource.com/c/go/+/638997
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>