[MRG] Convert nix to flakes

[luizirber]

Flakes are a new way of specifying Nix tasks (dev deps, package building, applications) and make it easier to be imported in other projects.

With a recent version on nix this already works:

nix run github:sourmash-bio/sourmash/lirber/nix_flake

and once the PR is merged then

nix run github:sourmash-bio/sourmash

will also work.

TODO

• Update CI
• Update docs

[codecov]

Codecov Report

Merging #1904 (b7fd3b9) into latest (1b6d9e1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           latest    #1904   +/-   ##
=======================================
  Coverage   82.75%   82.75%           
=======================================
  Files         122      122           
  Lines       13206    13206           
  Branches     1780     1780           
=======================================
  Hits        10928    10928           
  Misses       2014     2014           
  Partials      264      264           

Flag	Coverage Δ
python	90.65% <ø> (ø)
rust	65.80% <ø> (ø)

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[ctb]

did you mean to add Cargo.lock in this PR?

[luizirber]

did you mean to add Cargo.lock in this PR?

Yes, to make the builds reproducible (dependencies version-wise). The additional step is probably using --locked when calling cargo in setup.py, so the lockfile isn't updated automatically when building the Rust extension (and using dependabot for bumping versions)