Nix 0.30

[kanpov]

The classic Rust ecosystem quagmire: crate N updates from X to X+1, nothing else updates from X to X+1 (because X works) for ... weeks, so updating from X to X+1 is not viable for an individual crate since it would introduce a duplicate dependency on both X and X+1.

N = nix, X = 0.29 (not as bad as the quagmire caused by thiserror 1 -> 2).

Reminder to push out the update after ... amount of time when this "lovely" ecosystem updates.

[kanpov]

This bump might not even land in the final 0.7.0 release, interestingly enough.