flake: use explicit input format

And reorder the inputs to be in alphabetical order, because that is
always neater :-).

flake.lock

@@ -11,6 +11,7 @@
       },
       "original": {
         "owner": "numtide",
+        "ref": "master",
         "repo": "flake-utils",
         "type": "github"
       }
@@ -31,6 +32,7 @@
       },
       "original": {
         "owner": "nix-community",
+        "ref": "master",
         "repo": "home-manager",
         "type": "github"
       }
@@ -62,6 +64,7 @@
       },
       "original": {
         "owner": "nix-community",
+        "ref": "master",
         "repo": "NUR",
         "type": "github"
       }

flake.nix

@@ -1,14 +1,39 @@
 {
   description = "NixOS configuration with flakes";
   inputs = {
-    home-manager.url = "github:nix-community/home-manager";
-    home-manager.inputs.nixpkgs.follows = "nixpkgs";
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    nur.url = "github:nix-community/NUR";
-    futils.url = "github:numtide/flake-utils";
+    futils = {
+      type = "github";
+      owner = "numtide";
+      repo = "flake-utils";
+      ref = "master";
+    };
+
+    home-manager = {
+      type = "github";
+      owner = "nix-community";
+      repo = "home-manager";
+      ref = "master";
+      inputs = {
+        nixpkgs.follows = "nixpkgs";
+      };
+    };
+
+    nixpkgs = {
+      type = "github";
+      owner = "NixOS";
+      repo = "nixpkgs";
+      ref = "nixos-unstable";
+    };
+
+    nur = {
+      type = "github";
+      owner = "nix-community";
+      repo = "NUR";
+      ref = "master";
+    };
   };
 
-  outputs = { self, nixpkgs, nur, home-manager, futils } @ inputs:
+  outputs = { self, futils, home-manager, nixpkgs, nur } @ inputs:
     let
       inherit (futils.lib) eachDefaultSystem;