Fix NixOS system building and add a flake check that it works

Problem caused by: https://gerrit.lix.systems/c/lix/+/993

But this is actually nixpkgs doing extremely silly feature detection of
Nix based on version which it should /absolutely not be doing/.

flake.lock

@@ -36,11 +36,11 @@
     "lix": {
       "flake": false,
       "locked": {
-        "lastModified": 1714353561,
-        "narHash": "sha256-1OKjSB5kPw0Tnm/h4j1IZe6TfdX04QNbm1kn0q58Rls=",
+        "lastModified": 1714774053,
+        "narHash": "sha256-RaiwQCGFhC1WtieS2N3FotIlKSowq+R6ETyY6nZnPh8=",
         "ref": "refs/heads/main",
-        "rev": "c0733f3cec9e5edd7fa3a4bf156b6ee065762fea",
-        "revCount": 15448,
+        "rev": "549d609a7bce2b4ac28808a76a72d7f3743b9c76",
+        "revCount": 15471,
         "type": "git",
         "url": "ssh://git@git.lix.systems/lix-project/lix.git"
       },
@@ -51,11 +51,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1714253743,
-        "narHash": "sha256-mdTQw2XlariysyScCv2tTE45QSU9v/ezLcHJ22f0Nxc=",
+        "lastModified": 1714635257,
+        "narHash": "sha256-4cPymbty65RvF1DWQfc+Bc8B233A1BWxJnNULJKQ1EY=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "58a1abdbae3217ca6b702f03d3b35125d88a2994",
+        "rev": "63c3a29ca82437c87573e4c6919b09a24ea61b0f",
         "type": "github"
       },
       "original": {

flake.nix

@@ -28,5 +28,12 @@
         };
 
         packages.system-profile = import ./system-profile.nix { inherit pkgs flakey-profile; };
+
+        nixosTests = pkgs.recurseIntoAttrs (pkgs.callPackage ./test-nixos.nix { lix-module = self.nixosModules.default; });
+
+        checks = {
+          inherit (self.nixosTests.${system}) it-builds;
+          inherit (self.packages.${system}) default nix-eval-jobs;
+        };
       });
 }

overlay.nix

@@ -15,15 +15,15 @@ let
   })
   );
 
+  # This is kind of scary to not override the nix version to pretend to be
+  # 2.18 since nixpkgs can introduce new breakage in its Nix unstable CLI
+  # usage.
+  # https://github.com/nixos/nixpkgs/blob/6afb255d976f85f3359e4929abd6f5149c323a02/nixos/modules/config/nix.nix#L121
   lixPkg = (final.callPackage (lix + "/package.nix") {
     build-release-notes = false;
     versionSuffix = "-lix${versionSuffix}";
     boehmgc-nix = boehmgc-patched;
-  }).overrideAttrs {
-    # Note: load-bearing version override. Nixpkgs does version detection to determine
-    # what commands and whatnot we support, so tell Nixpkgs that we're 2.18 (ish).
-    version = "2.18.3-lix${versionSuffix}";
-  };
+  });
 
   inherit (prev) lib;
 in

test-nixos.nix

@@ -0,0 +1,17 @@
+{ nixos, lix-module }:
+let
+  configs = {
+    it-builds = nixos ({ ... }: {
+      documentation.enable = false;
+      fileSystems."/".device = "ignore-root-device";
+      boot.loader.grub.enable = false;
+      system.stateVersion = "24.05";
+    });
+
+  };
+in
+{
+  inherit configs;
+
+  it-builds = configs.it-builds.config.system.build.toplevel;
+}