Better support users/groups existing before install

[Hoverbear]

Description

This PR makes it so that if users/groups exist already on a machine when we attempt an install, and they're the UIDs/GIDs we expect, we mark them as already done. (This also means we don't remove them on uninstall)

Part of #35

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]

I think this looks fine.

[cole-h]

That's a weird type signature... I don't often see collecting into Result<_, _> 😆

[Hoverbear]

It's fun to can turn a Vec<Result<_,_>> into a Result<Vec<_>,_>

[Hoverbear]

This partially improves #218 but doesn't quite cover it.