chore: Fix failing unsoundness checks

[aborgna-q]

The unsoundness check was failing on main.

• Skips the hugr-cli tests when running miri, as it does not support calling external processes.
• Move impl Arbitrary for CustomSerialized out of a block of miri-skipped tests.
• Adds a just miri command with the correct flags to get proptests working with miri.

CI run: https://github.com/CQCL/hugr/actions/runs/9518163988/job/26238422543

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.58%. Comparing base (cbe31be) to head (8ad32d8).
Report is 1 commits behind head on main.

❗ Current head 8ad32d8 differs from pull request most recent head 51b90fc

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1196   +/-   ##
=======================================
  Coverage   86.58%   86.58%           
=======================================
  Files          94       94           
  Lines       17658    17658           
  Branches    16796    16796           
=======================================
  Hits        15289    15289           
  Misses       1600     1600           
  Partials      769      769           

Flag	Coverage Δ
rust	86.44% <100.00%> (ø)

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[doug-q]

Thanks!

[doug-q]

This is annoyingly non-uniform, everywhere else we have mod test::proptest, but it's fine. There's no reason to actually use this module anyway.

[aborgna-q]

Yeah, it's a bit annoying. But the alternative is copying the skip attribute to all the internal test functions, and that seems more bothersome.