flake: move 'checks' to 'flake/'

.envrc

@@ -3,4 +3,7 @@ if ! has nix_direnv_version || ! nix_direnv_version 2.2.1; then
 fi
 
 use flake
+
+watch_file ./flake/checks.nix
+
 eval "$shellHooks"

flake.nix

@@ -120,21 +120,7 @@
           default = apps.diff-flake;
         };
 
-        checks = {
+        checks = import ./flake/checks.nix inputs system;
-          pre-commit = pre-commit-hooks.lib.${system}.run {
-            src = ./.;
-
-            hooks = {
-              nixpkgs-fmt = {
-                enable = true;
-              };
-
-              shellcheck = {
-                enable = true;
-              };
-            };
-          };
-        };
 
         devShells = {
           default = pkgs.mkShell {

flake/checks.nix

@@ -0,0 +1,17 @@
+{ self, pre-commit-hooks, ... }:
+system:
+{
+  pre-commit = pre-commit-hooks.lib.${system}.run {
+    src = self;
+
+    hooks = {
+      nixpkgs-fmt = {
+        enable = true;
+      };
+
+      shellcheck = {
+        enable = true;
+      };
+    };
+  };
+}