nix: add build with eigen 5, ref #426

[nim65s]

This is currently failing, but should help to reproduce the issue.

[nim65s]

So in CI, we don't have timeouts:

2025-10-27T14:55:49.0419723Z        > 194/195 Test #142: dense maros meszaros using the api .....   Passed  986.58 sec
2025-10-27T14:55:49.0420424Z        > 195/195 Test #143: sparse maros meszaros using the API .....   Passed  1010.75 sec

With eigen 3, we had:

2025-10-27T14:15:38.2212389Z proxsuite> 194/195 Test #142: dense maros meszaros using the api .....   Passed   21.14 sec
2025-10-27T14:15:40.8429144Z proxsuite> 195/195 Test #143: sparse maros meszaros using the API .....   Passed   23.76 sec

So probably there is something wrong here :)

And this one still failed:

2025-10-27T14:55:49.0421861Z        > The following tests FAILED:
2025-10-27T14:55:49.0422182Z        >     66 - ProxQP::dense: test primal infeasibility solving (Failed)