Add a tgen nix Flake

[sporksmith]

This is mostly useful when using tgen as part of a larger nix configuration, e.g. to specify a reproducible environment for an experiment.

It can also be used to install tgen using the nix package manager. e.g. once merged, a user with the nix package manager installed could use: nix profile install github:shadow/tgen. (nix profile install github:sporksmith/tgen/flake should already work now)

[robgjansen]

We should add some documentation - at least a rough overview of how we might maintain the flake bits, since I'm going to assume most devs would not have much experience. (No need to re-review, unless you want another.)

[sporksmith]

Thanks! I asked eta @ tor if she could take a look since she has some experience with nix; I'll wait and see if she does before merging.

We should add some documentation - at least a rough overview of how we might maintain the flake bits

I intentionally avoided adding anything to the user docs yet since this is a bit of an experiment. We might decide next month or next week that nix isn't worth maintaining for us and rip it back out.

I'll make a pass to see if I can add some more comments about maintenance. Since this adds CI we can probably mostly ignore it if it doesn't break, though I suppose we ought to update the lock file and nix channel version occasionally.