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
eldritch horrors
35a2f28a46