`subprocess.run` docs should recommend copying `os.environ` on Windows

As described in https://stackoverflow.com/questions/78652758/, when passing custom environment variables to subprocess.run or subprocess.Popen on Windows, you will usually need to include the existing keys from os.environ in order to reliably run arbitrary processes (exactly which keys are required to be present depends on what the subprocess is doing).

The subprocess.Popen docs include a "Note" about this problem, but the subprocess.run docs omit it. Even the subprocess.Popen note is a bit vague, and only specifically mentions SystemRoot:

Note: If specified, env must provide any variables required for the program to execute. On Windows, in order to run a side-by-side assembly the specified env must include a valid SystemRoot.

The common documentation for the env parameter reads:

If env is not None, it must be a mapping that defines the environment variables for the new process; these are used instead of the default behavior of inheriting the current process’ environment. It is passed directly to Popen. This mapping can be str to str on any platform or bytes to bytes on POSIX platforms much like os.environ or os.environb.

Rather than re-using the existing subprocess.Popen note, my suggestion would be to add the following sentence to the above paragraph:

On Windows, many applications require specific environment variables (such as SystemRoot) to be set as expected in order to work correctly. Accordingly, it is recommended to start from os.environ.copy() when customing the subprocess environment on Windows, rather than starting from an empty dictionary (otherwise the executed subprocesses may fail with various Windows OS errors).

subprocess.run と subprocess.Popen の共有 env パラメータのドキュメントから始め、次に Popen ドキュメントにある既存の Windows に関する注記と比較します。Windows のサブプロセス環境をカスタマイズする際は os.environ.copy() から始めるよう案内を追加し、更新後の文言で SystemRoot などの必須変数が明確に扱われていることを確認します。