Continue moving personal packages from overlays

Continue moving out of the overlay model for everything and into running
sutff directly out of the packages. This should give a cleaner
separation of the things that I don't need to maintain separately

pkgs/inject-darwin.nix

@@ -0,0 +1,38 @@
+{
+  writeShellApplication,
+  coreutils,
+  curl,
+  gnutar,
+  nix,
+  ...
+}:
+writeShellApplication {
+  name = "inject-darwin";
+  runtimeInputs = [
+    coreutils
+    curl
+    gnutar
+    nix
+  ];
+  text = ''
+    dir="$(mktemp -d)"
+    cd "''${dir}"
+
+    # Install nix-darwin
+    nix-build https://github.com/LnL7/nix-darwin/archive/master.tar.gz -A installer
+    ./result/bin/darwin-installer
+
+    # Get my configuration
+    mkdir -p ~/.config/darwin
+    cd ~/.config/darwin
+    curl -O -L https://github.com/greg-hellings/nixos-config/archive/refs/heads/main.tar.gz
+    tar xvzf main.tar.gz --strip-components 1
+
+    # Build NixOS for this system
+    pushd "''${dir}"
+    nix build "~/.config/darwin#darwinConfigurations.$(hostname -s).system"
+    ./result/sw/bin/darwin-rebuild switch --flake ~/.config/darwin
+    popd
+    rm -r "''${dir}"
+  '';
+}