Load current_filename for Unstructured2D and P4est Meshes

[DanielDoehring]

There was an inconsistency between the mesh types: For Structured and TreeMesh the current_filename variable was correctly set, while for P4est and Unstructured2D this was not the case.

I discovered this when restarting a simulation, from which I saved solutions for which no mesh was specified, resulting in error when applying Trixi2Vtk.

[github-actions]

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[sloede]

can you maybe reopen this from a branch within Trixi.jl? Otherwise, can you please check if all errors are only codecov related?

[DanielDoehring]

See #1924