Bump the mach-nix source

[exarkun]

3.5.0 has fixes for a bug that surfaces when we use a sufficiently new nixpkgs.

See:

• https://github.com/DavHau/mach-nix/issues/467
• https://github.com/DavHau/mach-nix/issues/480

Also, our "direct reference" feature made it into upstream a while ago so we no longer need to use our fork.

[codecov[bot]]

Codecov Report

Merging #418 (f9b58d4) into main (fdc9836) will decrease coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #418      +/-   ##
==========================================
- Coverage   96.25%   96.22%   -0.03%     
==========================================
  Files          63       63              
  Lines        7525     7525              
  Branches     1012     1012              
==========================================
- Hits         7243     7241       -2     
- Misses        209      210       +1     
- Partials       73       74       +1     

Impacted Files	Coverage Δ
src/_zkapauthorizer/tahoe.py	92.61% <0.00%> (-1.48%)	:arrow_down:
src/_zkapauthorizer/tests/test_model.py	99.49% <0.00%> (-0.26%)	:arrow_down:
src/_zkapauthorizer/tests/test_client_resource.py	98.22% <0.00%> (+0.22%)	:arrow_up:
src/_zkapauthorizer/tests/_float_matchers.py	93.75% <0.00%> (+6.25%)	:arrow_up:

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