direnv.nix for direct envrc specification

[shlevy]

Might be nice to first check for direnv.nix, and if it exists just do a normal instantiate + realize (i.e. no IN_NIX_SHELL or anything) and take its $out as an envrc file directly.

[shlevy]

Or may be even better to teach direnv itself to somehow invoke an arbitrary command instead of "run bash sourcing this envrc and dumping env vars afterward", so we don't need to indirect through loading vars into the subshell just to specify what to change in the parent.