A wrapper script that takes ZFS snapshots prior to nixos-rebuild creating a new system

nixos-rebuild-zfs-snapshot.nix

This wrapper script creates a shell script named `nixos-rebuild-hooked` that reads your `sanoid` config for which datasets are being managed, snapshots them prior to `nixos-rebuild` running. If you are suing `flakes` like I am you can execute rebuild with these features: `nixos-rebuild-hooked switch --flake .`

```nix
{ config, pkgs, ... }:

{
  environment.systemPackages = with pkgs; [
    (pkgs.writeShellScriptBin "nixos-rebuild-hooked" ''
      echo "Running pre-rebuild hook..."

      # Get current system generation once
      CURRENT_GEN=$(ls -la /nix/var/nix/profiles/system | grep -o 'system-[0-9]*-link' | tail -1)
      echo "Current generation: $CURRENT_GEN"

      # Get list of datasets from sanoid configuration
      echo "Getting datasets from sanoid configuration..."
      DATASETS=$(sudo sanoid --verbose --debug --configdir=/etc/sanoid | grep Filesystem | awk '{print $2}')

      if [ -z "$DATASETS" ]; then
        echo "No datasets found in sanoid configuration"
        exit 1
      fi

      echo "Found datasets:"
      echo "$DATASETS"

      snapshot_with_prefix() {
        local dataset="$1"
        local timestamp=$(date +%Y-%m-%d_%H-%M-%S)
        local snapshot_name="$dataset@''${CURRENT_GEN}_$timestamp"

        echo "Creating snapshot: $snapshot_name"
        sudo zfs snapshot -r "$snapshot_name"
      }

      # Loop through each dataset and create snapshots
      while IFS= read -r dataset; do
        if [ -n "$dataset" ]; then
          snapshot_with_prefix "$dataset"
        fi
      done <<< "$DATASETS"

      echo "Pre-rebuild snapshots complete"

      # Run nixos-rebuild
      exec /run/current-system/sw/bin/nixos-rebuild "$@"
      '')
  ];
}
```