modernize nix a little

[michaelglass]

the problems

1. updating (or changing) the shell's veresion of librdkafka from nix is an arduous process
2. nix-shell is slow to launch

the solutions

1. niv handles nix versioning.

before

1. visit https://github.com/nixos/nixpkgs
2. find the right branch
3. find the right rev
4. download the rev and get the SHA of the download
5. update both in the right nix file.

after

$ niv update

2. [opt-in] if you have direnv installed and setup, direnv allow will cache nix-shell launch

this PR adds a .envrc, which in combination with direnv makes nix usage snappy

it also updates change the existing nixpkgs version to resolve security warnings with modern nixpkgs.

[AlexeyRaga]

Thanks, I'll have a look soon.
The diff looks scary though ;) We only need to provide a version of librdkafka in shell, and having that much nix code for it... Not sure how I feel about it, but I'll have a look :)

[michaelglass]

one of the files is the generated json parser from niv

[michaelglass]

@AlexeyRaga

• updated the PR description to explain the intent
• split up the commits so it's clear what part of this code was auto-generated by niv

[endgame]

@AlexeyRaga All this is doing is pinning nixpkgs instead of getting whatever version is in the user's channels. Until flakes take over (eventually, I hope, because they seem good in theory), niv seems to be the predominant way of pinning remote nix deps.

[AlexeyRaga]

@michaelglass There was a bit of a mess with branches (GH force-switched the default branch from master to main and I haven't noticed). I have update main, could you please update your PR too?

[michaelglass]

thanks @AlexeyRaga. rebased on main