Allocations in P4est2D calc_boundary_flux! for example euler_double_mach_amr

[DanielDoehring]

While tackling #1664 I observed that there are some allocations (most likely due to a type instability) in lines

Trixi.jl/src/solvers/dgsem_p4est/dg_2d.jl

Lines 336 to 341 in bb60b5d

	flux_ = boundary_condition(u_inner, normal_direction, x, t, surface_flux, equations)
	# Copy flux to element storage in the correct orientation
	for v in eachvariable(equations)
	surface_flux_values[v, node_index, direction_index, element_index] = flux_[v]
	end

See https://github.com/trixi-framework/Trixi.jl/actions/runs/6574426931/job/17859448565#step:7:3563