ci: fixes to runner image update

[ss2165]

• Updates devenv to match rust version on CI (1.84)
• clippy fixes
• Installs llvm all the time and removes separate hugr-llvm job because runner image no longer has LLVM to link to. Follow up issue: ci: try to avoid installing LLVM when not doing hugr-llvm tests #1851
• Nightly is failing for what appears to be an unrelated rustc bug. has been fixed!
• Semver checks failing because adding "llvm-14" to the apt-dependencies appears to not be changing anything??? https://github.com/CQCL/hugr/actions/runs/12707081975/job/35421268084?pr=1847 ci: Semver checks are broken #1852

Closes #1843

[hugrbot]

This PR contains breaking changes to the public Rust API.
Please deprecate the old API instead (if possible), or mark the PR with a ! to indicate a breaking change.

cargo-semver-checks summary

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.55%. Comparing base (b36d97d) to head (eb0c905).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1847   +/-   ##
=======================================
  Coverage   86.55%   86.55%           
=======================================
  Files         194      194           
  Lines       35156    35156           
  Branches    31969    31969           
=======================================
  Hits        30430    30430           
  Misses       2968     2968           
  Partials     1758     1758           

Flag	Coverage Δ
python	92.31% <ø> (ø)
rust	85.98% <100.00%> (ø)

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[doug-q]

Nightly and semver checks are failing, I think both are ICE in nightly rust.

I am approving, because if you think merging despite these failures is the best course then I trust your judgement.

Two suggestions two use the variables you defined.