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)

[nim65s]

As discussed in the meeting, I'm going to tell nix to ignore those particular failing tests, so that we can merge this, in order to ease the work on eigen5 support in dependent packages.
Those failures will still be tracked in #426