run.bash, cmd/dist: document GO_TEST_SHORT and GO_TEST_TIMEOUT_SCALE
    
    These environment variables affect cmd/dist, and in turn run.bash.
    They exist primarily for the Go build system, but are still needed
    sometimes when investigating problems. Document them in one place.
    
    Fixes #46054.
    
    Change-Id: I5dea2ac79b0d203d2f3c9ec2980382f62bead5cd
    Reviewed-on: https://go-review.googlesource.com/c/go/+/455517
    Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
    Reviewed-by: Bryan Mills <bcmills@google.com>
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
    Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>