Outdated Rust in nix

[kornelski]

@dpc Can you have a look at the CI? The nix setup has an older version of Rust, which keeps failing builds.

[dpc]

@kornelski Done, I think.

[kornelski]

Almost. Containers build can't build ./crevette. The error suggests that its src dir doesn't exist. Does it need allowlisting or excluding somewhere in nix?