[build2] Variables in nested config.build

[Boris Kolpackov]

Thanks for the testcase. Things were indeed working as you've
described because we read config.build during bootstrap (for
some convoluted reasons) which, unlike load, happens in the
inner-projects-first order.

That is definitely counter-intuitive so I've fixed this to read
config.build during load. If you grab the latest stage, your
test should work as expected.

Boris