ci: Pin nightly rust version

[aborgna-q]

Nightly rust tests are failing, seemingly due to rust-lang/rust#125474.

This PR fixes the toolchain to the last working nightly until the problem is resolved.

CI fail: https://github.com/CQCL/hugr/actions/runs/9281876942/job/25538729650#step:6:856

[aborgna-q]

Uhm, the ci-rs workflow is not getting triggered...

[aborgna-q]

I had replaced the uses: dtolnay/rust-toolchain@nightly line with uses: dtolnay/rust-toolchain and an explicit parameter for the nightly version,
but it seems that missing the branch name when specifying the action causes the workflow to silently fail (and not get triggered).

u.u

The fix was to add a @master prefix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.77%. Comparing base (f757cc8) to head (cbf6766).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1124      +/-   ##
==========================================
- Coverage   86.78%   86.77%   -0.01%     
==========================================
  Files          90       90              
  Lines       18546    18549       +3     
  Branches    18153    18156       +3     
==========================================
+ Hits        16095    16096       +1     
  Misses       1606     1606              
- Partials      845      847       +2     

Flag	Coverage Δ
rust	86.85% <ø> (-0.01%)	⬇️

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