flake.nix: Avoid some anti-patterns

Even though nixpkgs.legacyPackages isn’t directly an anti-pattern,
importing it allows passing non-default options.

flake.nix

@@ -55,13 +55,13 @@
     }@inputs: flake-utils.lib.eachDefaultSystem
       (system:
       let
-        pkgs = nixpkgs.legacyPackages.${system};
+        pkgs = import nixpkgs { inherit system; };
         inherit (pkgs) lib;
       in
       {
         checks = {
           pre-commit-check = nix-pre-commit-hooks.lib.${system}.run {
-            src = ./.;
+            src = self;
             hooks = {
               black.enable = true;
               nixpkgs-fmt.enable = true;
@@ -88,7 +88,7 @@
                   };
                   source = lib.evalSource (lib.singleton {
                     config.file = {
-                      path = toString ./.;
+                      path = toString self;
                       useChecksum = true;
                       filters = [
                         {