python: implement PEP 668 (#229166)

Add a marker file to the python outputs that tells pip and other tooling
following PEP 668 that they should not be installing things system-wide.

This provides better feedback to the user and also potentially avoids issues in
single-user installs where the /nix/store is owned by the user.

For more details, see <https://peps.python.org/pep-0668/>

Here is how it currently looks like:

    $ pip install requests
    error: externally-managed-environment

    × This environment is externally managed
    ╰─> This command has been disabled as it tries to modify the immutable
        `/nix/store` filesystem.

        To use Python with Nix and nixpkgs, have a look at the online documentation:
        <https://nixos.org/manual/nixpkgs/stable/#python>.

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