setup.py: honor CMAKE_GENERATOR_PLATFORM for the Windows -A flag

The Windows build hardcoded the Visual Studio target architecture to x64 for
any 64-bit interpreter (`if sys.maxsize > 2**32: -A x64`). On win-arm64 the
interpreter is 64-bit but the architecture is ARM, so this mis-targeted the
build and find_package(Python) then failed. Honor an explicit
CMAKE_GENERATOR_PLATFORM (e.g. "ARM64") from the environment, falling back to
the previous behavior otherwise.

Author: ax3l

setup.py

@@ -119,7 +119,10 @@ def build_extension(self, ext):
                     os.path.join(extdir, "openpmd_api")
                 )
             ]
-            if sys.maxsize > 2**32:
+            generator_platform = os.environ.get("CMAKE_GENERATOR_PLATFORM")
+            if generator_platform:
+                cmake_args += ['-A', generator_platform]
+            elif sys.maxsize > 2**32:
                 cmake_args += ['-A', 'x64']
             build_args += ['--', '/m']
         else: