flake: checks: convert to 'flake-parts'

flake/checks.nix

@@ -1,16 +1,20 @@
-{ self, pre-commit-hooks, ... }:
-system:
+{ self, inputs, ... }:
 {
-  pre-commit = pre-commit-hooks.lib.${system}.run {
-    src = self;
+  perSystem = { system, ... }: {
+    checks = {
+      # NOTE: seems like inputs' does not output the 'lib' attribute
+      pre-commit = inputs.pre-commit-hooks.lib.${system}.run {
+        src = self;
 
-    hooks = {
-      nixpkgs-fmt = {
-        enable = true;
-      };
+        hooks = {
+          nixpkgs-fmt = {
+            enable = true;
+          };
 
-      shellcheck = {
-        enable = true;
+          shellcheck = {
+            enable = true;
+          };
+        };
       };
     };
   };