build(nix): separate develop only flakes with a subflake

Inspired by NixOS/nix#6124

.envrc

@@ -1 +1 @@
-use flake
+use flake ./nix

.github/workflows/ci.yml

@@ -19,4 +19,4 @@ jobs:
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@v2
 
-      - run: nix flake check
+      - run: nix flake check ./nix

flake.nix

@@ -3,16 +3,11 @@
 
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
-    pre-commit-hooks = {
-      url = "github:cachix/pre-commit-hooks.nix";
-      inputs.nixpkgs.follows = "nixpkgs";
-    };
   };
 
   outputs = {
     self,
     nixpkgs,
-    pre-commit-hooks,
   }: let
     forEachSystem = nixpkgs.lib.genAttrs [
       "aarch64-darwin"
@@ -21,24 +16,6 @@
       "x86_64-linux"
     ];
   in {
-    devShells = forEachSystem (system: let
-      checks = {
-        pre-commit-check = pre-commit-hooks.lib.${system}.run {
-          src = ./.;
-          hooks = {
-            alejandra.enable = true;
-            prettier.enable = true;
-            shellcheck.enable = true;
-            shfmt.enable = true;
-          };
-        };
-      };
-    in {
-      default = nixpkgs.legacyPackages.${system}.mkShell {
-        inherit (checks.pre-commit-check) shellHook;
-      };
-    });
-
     packages = forEachSystem (system: {
       default = nixpkgs.legacyPackages.${system}.writeShellApplication {
         name = "git-diff-regex";

nix/flake.nix

@@ -0,0 +1,42 @@
+{
+  description = "git-diff-regex dev shell";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    pre-commit-hooks = {
+      url = "github:cachix/pre-commit-hooks.nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = {
+    self,
+    nixpkgs,
+    pre-commit-hooks,
+  }: let
+    forEachSystem = nixpkgs.lib.genAttrs [
+      "aarch64-darwin"
+      "aarch64-linux"
+      "x86_64-darwin"
+      "x86_64-linux"
+    ];
+  in {
+    devShells = forEachSystem (system: let
+      checks = {
+        pre-commit-check = pre-commit-hooks.lib.${system}.run {
+          src = ../.;
+          hooks = {
+            alejandra.enable = true;
+            prettier.enable = true;
+            shellcheck.enable = true;
+            shfmt.enable = true;
+          };
+        };
+      };
+    in {
+      default = nixpkgs.legacyPackages.${system}.mkShell {
+        inherit (checks.pre-commit-check) shellHook;
+      };
+    });
+  };
+}