Allow use of lix from nixpkgs

Change behaviour of module. Allow not specifying LixSrc
and provide fallback to nixpkgs.pkgs.lix

module.nix

@@ -1,5 +1,5 @@
-{ lix, versionSuffix ? "" }:
+{ lixSrc ? null, versionSuffix ? "" }:
 { pkgs, config, ... }:
 {
-  nixpkgs.overlays = [ (import ./overlay.nix { inherit lix versionSuffix; }) ];
+  nixpkgs.overlays = [ (import ./overlay.nix { inherit lixSrc versionSuffix; }) ];
 }

overlay.nix

@@ -1,11 +1,15 @@
-{ lix, versionSuffix ? "" }:
+{ lixSrc ? null, versionSuffix ? "" }:
 final: prev:
 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") ({
+  lixPkg =
+    if lixSrc == null
+    then prev.pkgs.lix
+    else
+      final.callPackage (lixSrc + "/package.nix") ({
         versionSuffix = "-${versionSuffix}";
         # FIXME: do this more sensibly for future releases
         # https://git.lix.systems/lix-project/lix/issues/406