use cross from LinearAlgebra instead of Trixi.cross

[ranocha]

This avoids possible issues if we change something in Trixi.jl.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.45%. Comparing base (13611fc) to head (bf12c3d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #63   +/-   ##
=======================================
  Coverage   89.45%   89.45%           
=======================================
  Files          20       20           
  Lines        1802     1802           
=======================================
  Hits         1612     1612           
  Misses        190      190           

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[tristanmontoya]

This makes sense; LGTM. The windows CI errors are unrelated, but haven't looked into the specific cause (they seem to be present in Trixi.jl as well).