chore: nix and pixi updates

[luizirber]

Update nix and pixi deps

Add a pixi task to check MSRV

Move [metadata] table into [tool.sourmash.metadata], according to https://packaging.python.org/en/latest/specifications/pyproject-toml/ it is an error to have it at top-level:

The pyproject.toml file is written in TOML. Three tables are currently specified, namely [build-system], [project] and [tool]. Other tables are reserved for future use (tool-specific configuration should use the [tool] table).

Fix license to follow https://peps.python.org/pep-0639/
Skip license-file for now because maturin errors out

Use nextest to run tests locally

Add Python 3.13 as supported version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.02%. Comparing base (af534a9) to head (9a04f3b).

Additional details and impacted files

@@           Coverage Diff           @@
##           latest    #3508   +/-   ##
=======================================
  Coverage   88.02%   88.02%           
=======================================
  Files         137      137           
  Lines       22312    22312           
  Branches     2225     2225           
=======================================
  Hits        19640    19640           
  Misses       2359     2359           
  Partials      313      313           

Flag	Coverage Δ
hypothesis-py	25.43% <ø> (ø)
python	92.32% <ø> (ø)
rust	82.07% <ø> (ø)

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