run.{bash,bat,rc}: use ../bin/go instead of the go binary in $PATH

https://golang.org/doc/contribute.html#quick_test currently suggests
running 'make.bash' and 'run.bash' separately, but 'run.bash'
potentially uses a 'go' command resolved from the wrong GOROOT,
which in turn sets the wrong GOROOT for further commands.

Updates #32674
Updates #17896

Change-Id: I4925d478d0fc7351c4f6d40830ab17d4d688348d
Reviewed-on: https://go-review.googlesource.com/c/go/+/223741


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>