allow to nix run nixos-shells from flakes

now it's possible to do this:

```nix
{
  imports = [
    inputs.nixos-shell.nixosModules.nixos-shell
  ];
}

For the pure version (doesn't set SHELL or mount /home):

```
nix run .#nixosConfigurations.installer.config.system.build.nixos-shell
```

Or for a version closer to `nixos-shell`:

```
nix run .#nixosConfigurations.myconfig.config.system.build.nixos-shell
```

Author: Mic92

README.md

@@ -67,6 +67,7 @@ If an attribute _name_ is given, `nixos-shell` tries the following flake output
 - `nixosConfigurations.<name>`
 - `nixosModules.<name>`
 
+
 ## Terminating the virtual machine
 
 Type `Ctrl-a x` to exit the virtual machine.
@@ -247,6 +248,40 @@ To avoid having to download a nix-channel every time the VM is reset, you can us
 
 This will add the nixpkgs that is used for the VM in the NIX_PATH of login shell.
 
+## Embedding nixos-shell in your own nixos-configuration
+
+Instead of using the cli, it's also possible to include the `nixos-shell` NixOS module in your own NixOS configuration.
+
+Add this to your `flake.nix`:
+
+```nix
+{
+  inputs.nixos-shell.url = "github:Mic92/nixos-shell";
+}
+```
+
+And this to your nixos configuration defined in your flake:
+
+```nix
+{
+  imports = [ inputs.nixos-shell.nixosModules.nixos-shell ];
+}
+```
+
+Afterwards you can start your nixos configuration with nixos-shell with one of the two following variants:
+
+For the pure version (doesn't set SHELL or mount /home):
+
+```
+nix run .#nixosConfigurations.<yourmachine>.config.system.build.nixos-shell
+```
+
+Or for a version closer to `nixos-shell`:
+
+```
+nix run .#nixosConfigurations.<yourmachine>.config.system.build.nixos-shell
+```
+
 ## More configuration
 
 Have a look at the [virtualisation] options NixOS provides.

share/modules/nixos-shell.nix

@@ -1,4 +1,4 @@
-{ lib, modulesPath, ... }:
+{ lib, modulesPath, pkgs, extendModules, ... }:
 
 {
   imports = [
@@ -59,4 +59,14 @@
       };
     };
   };
+
+  config = let
+    vmSystem = extendModules {
+      modules = [
+        ./nixos-shell-config.nix
+      ];
+    };
+  in {
+    system.build.nixos-shell = vmSystem.config.system.build.vm;
+  };
 }