api: Fix custom fd for staggered #6357

Job	Run time
pytest-ubuntu-py311-gcc11-noomp-base	28m 47s
pytest-ubuntu-py311-gcc11-noomp-adjoint	27m 26s
pytest-ubuntu-py38-gcc12-omp-base	28m 41s
pytest-ubuntu-py38-gcc12-omp-adjoint	27m 17s
pytest-ubuntu-py37-gcc7-omp-base	29m 42s
pytest-ubuntu-py37-gcc7-omp-adjoint	28m 38s
pytest-ubuntu-py310-gcc10-noomp-base	27m 48s
pytest-ubuntu-py310-gcc10-noomp-adjoint	27m 15s
pytest-ubuntu-py312-gcc13-omp-base	40m 26s
pytest-ubuntu-py312-gcc13-omp-adjoint	38m 52s
pytest-ubuntu-py39-gcc9-omp-base	28m 25s
pytest-ubuntu-py39-gcc9-omp-adjoint	27m 36s
pytest-osx-py37-clang-omp-base	3m 15s
pytest-docker-py39-gcc-omp-base	29m 58s
pytest-docker-py39-gcc-omp-adjoint	25m 32s
pytest-docker-py39-icx-omp-base	25m 23s
pytest-docker-py39-icx-omp-adjoint	26m 21s
	7h 51m 22s

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