use extendModules to add nixos-shell modules

if `extendModules` is available on the supplied NixOS configuration, in
preference to using `override`.

This permits callers to omit `makeOverridable` when using a
recent-enough nixpkgs.

Author: tomeon

README.md

@@ -39,7 +39,11 @@ $ nixos-shell --flake github:Mic92/nixos-shell#vm-forward
 
 This will run the `vm-forward` example.
 
-> Note: system configurations have to be made overridable with `lib.makeOverridable` to use them with `nixos-shell`
+> Note: `nixos-shell` must be able to extend the specified system configuration with [certain modules](share/modules).
+>
+> If your version of `nixpkgs` provides the `extendModules` function on system configurations, `nixos-shell` will use it to inject the required modules; no additional work on your part is needed.
+>
+> If your version of `nixpkgs` **does not** provide `extendModules`, you must make your system configurations overridable with `lib.makeOverridable` to use them with `nixos-shell`:
 >```nix
 >{
 >  nixosConfigurations = let
@@ -51,6 +55,7 @@ This will run the `vm-forward` example.
 >  };
 >}
 >```
+> Specifying a non-overridable system configuration will cause `nixos-shell` to abort with a non-zero exit status.
 
 When using the `--flake` flag, if no attribute is given, `nixos-shell` tries the following flake output attributes:
 - `packages.<system>.nixosConfigurations.<vm>`

share/nixos-shell.nix

@@ -31,21 +31,19 @@ let
     (getFlakeOutput [ "packages" "${system}" "nixosConfigurations" "${flakeAttr}" ]);
 
   flakeModule = getFlakeOutput [ "nixosModules" "${flakeAttr}" ];
+
+  nixosShellModules =
+    if flakeSystem ? options.nixos-shell then
+      [ nixos-shell-config ]
+    else
+      [ nixos-shell nixos-shell-config ];
 in
 if flakeUri != null then
   if flakeSystem != null then
-    flakeSystem.override
-      (attrs: {
-        modules =
-          let
-            nixosShellModules =
-              if flakeSystem ? options.nixos-shell then
-                [ nixos-shell-config ]
-              else
-                [ nixos-shell nixos-shell-config ];
-          in
-          attrs.modules ++ nixosShellModules;
-      })
+    if flakeSystem ? "extendModules" then
+      flakeSystem.extendModules { modules = nixosShellModules; }
+    else
+      flakeSystem.override (attrs: { modules = attrs.modules ++ nixosShellModules; })
   else if flakeModule != null then
     mkShellSystem flakeModule
   else