chore(nix): Bump nixpkgs and install nodejs_20 to keep pre-commit happy

I was getting the following error:

```
An unexpected error has occurred: CalledProcessError: command: ('/nix/store/15jzs4a11nqp4m1xvnw0rz9395anzjsm-nodejs-18.20.8/bin/node', '/run/current-system/sw/bin/npm', 'install', '--include=dev', '--include=prod', '--ignore-prepublish', '--no-progress', '--no-save')
return code: 1
stdout: (none)
stderr:
    npm error code EBADENGINE
    npm error engine Unsupported engine
    npm error engine Not compatible with your version of node/npm: [email protected]
    npm error notsup Not compatible with your version of node/npm: [email protected]
    npm error notsup Required: {"node":">=20"}
    npm error notsup Actual:   {"npm":"10.8.2","node":"v18.20.8"}
    npm error A complete log of this run can be found in: /home/nick/.npm/_logs/2025-05-28T10_40_07_463Z-debug-0.log
Check the log at /home/nick/.cache/pre-commit/pre-commit.log
```

Author: NickLarsenNZ

nix/sources.json

@@ -1,6 +1,7 @@
-{ sources ? import ./nix/sources.nix
-, nixpkgs ? sources.nixpkgs
-, pkgs ? import nixpkgs { }
+{
+  sources ? import ./nix/sources.nix,
+  nixpkgs ? sources.nixpkgs,
+  pkgs ? import nixpkgs { },
 }:
 
 let
@@ -9,6 +10,7 @@ in
 pkgs.mkShell {
   packages = [
     bake
+    pkgs.nodejs_20
   ];
 
   buildInputs = [