Add pre install/uninstall checks to planners

[Hoverbear]

Description

Add structured pre install/uninstall planner steps to mitigate users encountering #528.

Testing

ephemeraladmin@mac-epic-turducken ~ % curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/561 | sh -s -- install

ephemeraladmin@mac-epic-turducken ~ % . /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh

Create a flake.nix with these contents in some folder (mine is boop, then cd in:

{
  description = "John's darwin system";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-23.05-darwin";
    darwin.url = "github:lnl7/nix-darwin/master";
    darwin.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = inputs@{ self, darwin, nixpkgs }: {
    darwinConfigurations."mac-epic-turducken" = darwin.lib.darwinSystem {
      modules = [(
        { pkgs, ... }:
        {
          # List packages installed in system profile. To search by name, run:
          # $ nix-env -qaP | grep wget
          environment.systemPackages =
            [ pkgs.vim
            ];

          # Auto upgrade nix package and the daemon service.
          services.nix-daemon.enable = true;
          nix.package = pkgs.nix;
          nixpkgs.hostPlatform = "aarch64-darwin";
        }
      )];
    };
  };
}

Doing the rest:

ephemeraladmin@mac-epic-turducken boop % nix build .#darwinConfigurations.mac-epic-turducken.system

ephemeraladmin@mac-epic-turducken boop % printf 'run\tprivate/var/run\n' | sudo tee -a /etc/synthetic.conf

ephemeraladmin@mac-epic-turducken boop % sudo /System/Library/Filesystems/apfs.fs/Contents/Resources/apfs.util -t


ephemeraladmin@mac-epic-turducken boop % sudo ./result/sw/bin/darwin-rebuild switch --flake ~/boop               
warning: $HOME ('/Users/ephemeraladmin') is not owned by you, falling back to the one defined in the 'passwd' file ('/var/root')
building the system configuration...
warning: $HOME ('/Users/ephemeraladmin') is not owned by you, falling back to the one defined in the 'passwd' file ('/var/root')
warning: $HOME ('/Users/ephemeraladmin') is not owned by you, falling back to the one defined in the 'passwd' file ('/var/root')
user defaults...
setting up user launchd services...
setting up /Applications/Nix Apps...
setting up pam...
applying patches...
setting up /etc...
error: not linking environment.etc."bashrc" because /etc/bashrc already exists, skipping...
existing file has unknown content 08ffbf991a9e25839d38b80a0d3bce3b5a6c84b9be53a4b68949df4e7e487bb7, move and activate again to apply
error: not linking environment.etc."nix/nix.conf" because /etc/nix/nix.conf already exists, skipping...
existing file has unknown content 9b5b5aedd302e950e450a28d6d42cc087bfd1c40951c4617096b9b12b2007c5f, move and activate again to apply
system defaults...
setting up launchd services...
creating service org.nixos.activate-system
reloading service org.nixos.nix-daemon
reloading nix-daemon...
waiting for nix-daemon
waiting for nix-daemon
configuring networking...

ephemeraladmin@mac-epic-turducken boop % ./nix-installer uninstall
`nix-installer` needs to run as `root`, attempting to escalate now via `sudo`...
`nix-darwin` installation detected, it must be removed before uninstalling Nix. Please refer to https://github.com/LnL7/nix-darwin#uninstalling for instructions how to uninstall `nix-darwin`

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 good!