chore: make cythonize incremental by default (#13676)

We make the cythonisation of pyx sources incremental by default. The old
behaviour whereby all Cython sources are forcibly re-transpiled can be
recovered by setting `DD_SETUP_FORCE_CYTHONIZE=1`.

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Author: P403n1x87

setup.py

@@ -842,7 +842,7 @@ def get_exts_for(name):
             "PY_MICRO_VERSION": sys.version_info.micro,
             "PY_VERSION_HEX": sys.hexversion,
         },
-        force=True,
+        force=os.getenv("DD_SETUP_FORCE_CYTHONIZE", "0") == "1",
         annotate=os.getenv("_DD_CYTHON_ANNOTATE") == "1",
         compiler_directives={"language_level": "3"},
     )