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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