cmd/go: make TestScript/gotoolchain more realistic

cmd/go: make TestScript/gotoolchain more realistic

- Build the fake go1.999testpath binary from Go source instead of
  special-casing a fake command on Windows.

- Skip the part of the test that uses shell scripts served from the
  test GOPROXY if /bin/sh is not present.

This makes the test more expensive, but also more realistic: notably,
it does not require test hooks to determine whether to run a real or
fake binary.

Change-Id: If14fec52186631d7833eba653c91ec5198dede58
Reviewed-on: https://go-review.googlesource.com/c/go/+/486400
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>