tvix: be able to specify a (nix file, attribute path) as a GC root

Opened by kanepyork at 2020-11-27T19·07+00

We should be able to, as part of developer setup, make the current dependencies of all (or a configurable subset of) depot packages treated as reachable by the nix GC.

This would greatly simplify the task of collecting garbage after a nixpkgs channel upgrade.