[BUG]: Overwriting CMake PYTHON_MODULE_EXTENSION needs PYBIND11_PYTHON_EXECUTABLE_LAST

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Problem description

I realized when trying a work-around for Conda-Forge, which requires me to overwrite PYTHON_MODULE_EXTENSION for PyPy3.7, that I cannot set this variable from the command line without also setting PYBIND11_PYTHON_EXECUTABLE_LAST to the same value as Python_EXECUTABLE (on a fresh build directory).
https://github.com/conda-forge/openpmd-api-feedstock/pull/86

I cannot spot the reason why in pybind11NewTools.cmake or the source of CMake for unset(.. CACHE), yet. But it seems that a passed -DPYTHON_MODULE_EXTENSION=... is unconditionally overwritten.

Reproducible example code

Any build that adds -DPython_EXECUTABLE:FILEPATH=$PYTHON -DPython_INCLUDE_DIR=$(${PYTHON} -c "from sysconfig import get_paths as gp; print(gp()['include'])") -DPYTHON_MODULE_EXTENSION=".something-else.so" as of pybind11 2.8.1 or 2.9.0 (CMake: 3.21.3 & 3.22.1).
I see this with openPMD-api 0.14.4:

• https://github.com/openPMD/openPMD-api
• https://github.com/conda-forge/openpmd-api-feedstock/pull/86

cmake -S . -B build -DopenPMD_USE_PYTHON=ON -DPython_EXECUTABLE:FILEPATH=$(which python3) -DPython_INCLUDE_DIR=$($(which python3) -c "from sysconfig import get_paths as gp; print(gp()['include'])") -DopenPMD_USE_INTERNAL_PYBIND11=OFF -DPYTHON_MODULE_EXTENSION=".something-else.so"
cmake --build build -j 4

# pybind11 2.6.1 + CMake 3.22.1
ls build/lib/python3*/site-packages/openpmd_api/
openpmd_api_cxx.something-else.so

# pybind11 2.8.1 + CMake 3.22.1 or
# pybind11 2.9.0 + CMake 3.21.3 (conda-forge)
ls build/lib/python3*/site-packages/openpmd_api/
openpmd_api_cxx.cpython-39-x86_64-linux-gnu.so