feat(meta): Remove default values for nixpkgs version

They are considered harmful, and it simplifies the code.

meta/README.md

@@ -32,7 +32,7 @@ TODO.
 
 # Nixpkgs
 
-Machines can use different versions of NixOS, the supported and default ones are specified here.
+Machines can use different versions of NixOS, the supported ones are specified here.
 
 ## How to add a new version
 
@@ -55,7 +55,7 @@ The nodes are declared statically, several options can be configured:
 
 - `deployment`, the colmena deployment option
 - `stateVersion`, the state version of the node
-- `nixpkgs`, the version of NixOS to use
+- `nixpkgs`, the version and sytem of NixOS to use
 - `admins`, the list of administrators specific to this node, they will be given root access
 - `adminGroups`, a list of groups whose members will be added to `admins`
 - `site`, the physical location of the node

meta/nixpkgs.nix

@@ -1,25 +1,15 @@
 {
-  versions = {
-    # Default version of nixpkgs to use
-    default = "24.05";
-
+  versions = [
     # Supported nixpkgs versions
-    supported = [
-      "unstable"
-      "23.11"
-      "24.05"
-      "24.11"
-    ];
-  };
-
-  systems = {
-    # Default system for our deployments
-    default = "nixos";
+    "unstable"
+    "23.11"
+    "24.05"
+    "24.11"
+  ];
 
+  systems = [
     # Supported system types
-    supported = [
-      "zyxel-nwa50ax"
-      "nixos"
-    ];
-  };
+    "zyxel-nwa50ax"
+    "nixos"
+  ];
 }

meta/options.nix

@@ -141,16 +141,15 @@ in
 
               nixpkgs = {
                 version = mkOption {
-                  type = enum nixpkgs.versions.supported;
-                  inherit (nixpkgs.versions) default;
+                  type = enum nixpkgs.versions;
                   description = ''
                     Version of nixpkgs to use.
                   '';
+                  example = "unstable";
                 };
 
                 system = mkOption {
-                  type = enum nixpkgs.systems.supported;
-                  inherit (nixpkgs.systems) default;
+                  type = enum nixpkgs.systems;
                   description = ''
                     Type of system for this node, will impact how it is evaluated and deployed.
                   '';