Disallow reusing the command to launch two kernels

[grospelliergilles]

It was never allowed but there was no test to check it.
Re-using the same command will make profiling results wrong and may also have unknown side effects.
We can temporarily set the environment variable ARCANE_ACCELERATOR_ALLOW_REUSE_COMMAND to 1 allows to revert to previous behavior.

[codecov]

Codecov Report

Attention: Patch coverage is 91.89189% with 9 lines in your changes missing coverage. Please review.

Project coverage is 70.22%. Comparing base (6510f63) to head (2e23286).
Report is 5 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...cane/src/arcane/accelerator/core/RunCommandImpl.cc	0.00%	6 Missing and 2 partials ⚠️
...cane/tests/accelerator/AcceleratorViewsUnitTest.cc	97.87%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1790      +/-   ##
==========================================
- Coverage   70.24%   70.22%   -0.03%     
==========================================
  Files        2258     2258              
  Lines      162343   162364      +21     
  Branches    18698    18703       +5     
==========================================
- Hits       114036   114017      -19     
- Misses      41537    41572      +35     
- Partials     6770     6775       +5     

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