Make stubgen respect MYPY_CACHE_DIR

This is required when using stubgen in a parallel build system, such
as GNU make.

mypy/stubgen.py

@@ -1588,6 +1588,13 @@ def mypy_options(stubgen_options: Options) -> MypyOptions:
     options.python_version = stubgen_options.pyversion
     options.show_traceback = True
     options.transform_source = remove_misplaced_type_comments
+
+    # Override cache_dir if provided in the environment
+    environ_cache_dir = os.getenv("MYPY_CACHE_DIR", "")
+    if environ_cache_dir.strip():
+        options.cache_dir = environ_cache_dir
+    options.cache_dir = os.path.expanduser(options.cache_dir)
+
     return options
 
 
@@ -1751,6 +1758,9 @@ def generate_stubs(options: Options) -> None:
 manual changes.  This directory is assumed to exist.
 """
 
+FOOTER: Final = """Environment variables:
+  Define MYPY_CACHE_DIR to override configuration cache_dir path."""
+
 
 def parse_options(args: list[str]) -> Options:
     parser = argparse.ArgumentParser(prog="stubgen", usage=HEADER, description=DESCRIPTION)