Calling `go env GOROOT` and `go version` will try to download a newer
toolchain if the current one is too old. This is not what we want when
building the toolchain itself. Avoid this by setting `GO111MODULE=off`.
While here, align the unset variables with nogoenv(), present in
make.bash and make.rc.
Fixes #62201
Fixes #62281
Change-Id: I2d3ac817c2fb161f7a31bf83e32ba934e773b6c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/523015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com> Reviewed-by: Bryan Mills <bcmills@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>