[build2] Using ssh-git

[Boris Kolpackov]

Boris Kolpackov <boris at codesynthesis.com> writes:

> We will try to add ssh:// support for the next release. I will ping you
> when we have something for you to try.

I've added support for this for the upcoming release. It would be great
if you could try the staged toolchain and confirm it works for you. It's
the same installation process, just the install scripts come from this
location:

https://stage.build2.org/0/