This absorbs a josh-filtered Nix subtree into depot, at
//tools/nixery.
This subtree was created through `josh-filter ':prefix=tools/nixery'`,
which allows a filter on tools/nixery to yield the same commit hashes
as the original Nixery repository (allowing for history continuity).
Change-Id: Icc1a99bf1248226b91f437b0a90361d36fb0d327
