Document the known nix-darwin related uninstall issue

[Hoverbear]

Description

Until we add some knowledge to the macos planner about how to do this automatically, this is a stopgap that uses the workaround presented by @cole-h in #608 (comment).

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[cole-h]

after my typo fix above, LGTM