src: emit warning when make.bash run with invalid GOROOT_BOOTSTRAP
When setting an invalid GOROOT_BOOTSTRAP, make.bash try looking at env to
find a valid go command and use that GOROOT instead. It's better if the
script emit a warning, instead of silently change the GOROOT_BOOTSTRAP.
Change-Id: I6038093d5e8c072da27bcae44b125f6774f809a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/326030
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>