Set multiprocessing start method only if not already set

[TheJostler]

I was getting this error when trying to build the project for the first time using the ./configure --download-nyan command:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/josj/scr/openage/.bin/g++-debug-Oauto-sanitize-none/openage/__main__.py", line 178, in <module>
    multiprocessing.set_start_method('spawn')
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/context.py", line 247, in set_start_method
    raise RuntimeError('context has already been set')
RuntimeError: context has already been set
CMake Error at buildsystem/codegen.cmake:32 (message):
  failed to get target list from codegen invocation
Call Stack (most recent call first):
  buildsystem/python.cmake:495 (codegen_run)
  CMakeLists.txt:261 (python_finalize)

It required a very simple fix, just added a check before setting the multiprocessing start method to ensure that it only gets set once. After this change was made to the build script worked perfectly.