ci: fix model runs

[lkstrp]

See PyPSA/PyPSA#1443

[lkstrp]

Hey @brynpickering,

I played around here and need your pixi help again. We can't do the same as in PyPSA/PyPSA#1443, since linopy is a dependency of pypsa. And pixi allows to remove it (why?) but does not allow to install it again. There is no --no-deps flag, which I would normally use and removing pypsa first and bringing it back after the linopy install, is hacky and does not work, since we lose the pinned range defined in eur then. There must be a proper way to handle this. Any ideas?

We could also remove it completely. But we had a serious bug introduced in a linopy a couple of months ago, which the model run tests would have caught.