implementing a build hook is pretty much impossible without either being
a nix, or blindly forwarding the important bits of all build requests to
some kind of nix. we've found no uses of build-hook in the wild, and the
build-hook protocol (apart from being entirely undocumented) is not able
to convey any kind of versioning information between hook and daemon. if
we want to upgrade this infrastructure (which we do), this must not stay
Change-Id: I1ec4976a35adf8105b8ca9240b7984f8b91e147e
