chore: Fix unsoundness tests

[aborgna-q]

A test was using the filesystem to check insta snapshots. This is not supported by miri.

drive-by: Fix lint warning
drive-by: Don't send a slack notification when running the miri checks manually on an arbitrary commit.

CI run:
https://github.com/CQCL/hugr/actions/runs/11726988593

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.79%. Comparing base (3a297d5) to head (9a40e90).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1643      +/-   ##
==========================================
- Coverage   85.79%   85.79%   -0.01%     
==========================================
  Files         136      136              
  Lines       25180    25180              
  Branches    22092    22092              
==========================================
- Hits        21604    21603       -1     
- Misses       2426     2427       +1     
  Partials     1150     1150              

Flag	Coverage Δ
python	92.42% <ø> (ø)
rust	84.86% <100.00%> (-0.01%)	⬇️

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