Update Nix installation instructions

[flexxyfluxx]

see #330

[printfn]

Thanks for the suggestion. I guess it makes sense not to recommend nix-env, but I do wonder if maybe we can make it a bit shorter. I feel like people using nix would already know how to install packages on their system, so maybe we could just say it's in nixpkgs, or maybe provide the nix-shell command?

[flexxyfluxx]

You're right, it is long. The bit about why nix-env is bad is probably also unnecessary, those who would possibly be benefitted by having access to that definitely don't need telling it exists.

[flexxyfluxx]

i've shortened it, lmk what you think. seems to fit on a single line now (at least in a browser, on a desktop)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.58%. Comparing base (b838c78) to head (96c47d4).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #331   +/-   ##
=======================================
  Coverage   82.58%   82.58%           
=======================================
  Files          52       52           
  Lines       14364    14364           
=======================================
  Hits        11863    11863           
  Misses       2501     2501           

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[printfn]

Thanks, looks good!

[flexxyfluxx]

At last, I have found the button to open this PR back up.