fix allocations in boundary condition of Euler double Mach elixir

[ranocha]

Closes #1680

[github-actions]

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[DanielDoehring]

Ah I see, good catch!

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (c2f0095) 82.63% compared to head (7ea13e2) 82.53%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1685      +/-   ##
==========================================
- Coverage   82.63%   82.53%   -0.10%     
==========================================
  Files         425      425              
  Lines       34376    34399      +23     
==========================================
- Hits        28406    28389      -17     
- Misses       5970     6010      +40     

Flag	Coverage Δ
unittests	82.53% <0.00%> (-0.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...les/p4est_2d_dgsem/elixir_euler_double_mach_amr.jl	0.00% <0.00%> (ø)

... and 230 files with indirect coverage changes

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