fix(nix): fix shell.nix for non flake nix-shell invocation

- relocated shell.nix to nix/shell.nix
- initialized shell.nix at root to correctly invoke the shell with
  `nix-shell` command for non flake

flake.nix

@@ -42,7 +42,7 @@
       system: let
         pkgs = nixpkgs.legacyPackages.${system};
       in {
-        default = pkgs.callPackage ./shell.nix {};
+        default = pkgs.callPackage ./nix/shell.nix {};
       }
     );
 

nix/shell.nix

@@ -0,0 +1,31 @@
+{
+  alejandra,
+  statix,
+  deadnix,
+  shfmt,
+  shellcheck,
+  jsonfmt,
+  lefthook,
+  kdePackages,
+  mkShellNoCC,
+}:
+mkShellNoCC {
+  #it's faster than mkDerivation / mkShell
+  packages = [
+    # nix
+    alejandra # formatter
+    statix # linter
+    deadnix # linter
+
+    # shell
+    shfmt # formatter
+    shellcheck # linter
+
+    # json
+    jsonfmt # formatter
+
+    # CoC
+    lefthook # githooks
+    kdePackages.qtdeclarative # qmlfmt, qmllint, qmlls and etc; Qt6
+  ];
+}

shell.nix

@@ -1,31 +1 @@
-{
+(import <nixpkgs> {}).callPackage ./nix/shell.nix {}
-  alejandra,
-  statix,
-  deadnix,
-  shfmt,
-  shellcheck,
-  jsonfmt,
-  lefthook,
-  kdePackages,
-  mkShellNoCC,
-}:
-mkShellNoCC {
-  #it's faster than mkDerivation / mkShell
-  packages = [
-    # nix
-    alejandra # formatter
-    statix # linter
-    deadnix # linter
-
-    # shell
-    shfmt # formatter
-    shellcheck # linter
-
-    # json
-    jsonfmt # formatter
-
-    # CoC
-    lefthook # githooks
-    kdePackages.qtdeclarative # qmlfmt, qmllint, qmlls and etc; Qt6
-  ];
-}