flake: fix foreman execution

[vin@scadrial:~/workspace/vcs/hydra]$ foreman -h
    Warning: the running version of Bundler (2.1.4) is older than the version that created the lockfile (2.2.20). We suggest you to upgrade to the version that created the lockfile by running `gem install bundler:2.2.20`.
    Traceback (most recent call last):
    	2: from /nix/store/ycshcdssxcj9sjf6yzb1ydw4fcglf66y-foreman-0.87.2/bin/foreman:20:in `<main>'
    	1: from /nix/store/ggqacj06n6qfm1iww0bih9ph0j89wcna-bundler-2.1.4/lib/ruby/gems/2.7.0/gems/bundler-2.1.4/lib/bundler/rubygems_integration.rb:413:in `block in replace_bin_path'
    /nix/store/ggqacj06n6qfm1iww0bih9ph0j89wcna-bundler-2.1.4/lib/ruby/gems/2.7.0/gems/bundler-2.1.4/lib/bundler/rubygems_integration.rb:374:in `block in replace_bin_path': can't find executable foreman for gem foreman. foreman is not currently included in the bundle, perhaps you meant to add it to your Gemfile? (Gem::Exception)

flake.lock

@@ -16,6 +16,22 @@
         "type": "github"
       }
     },
+    "newNixpkgs": {
+      "locked": {
+        "lastModified": 1646588256,
+        "narHash": "sha256-ZHljmNlt19nSm0Mz8fx6QEhddKUkU4hhwFmfNmGn+EY=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "2ebb6c1e5ae402ba35cca5eec58385e5f1adea04",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-21.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nix": {
       "inputs": {
         "lowdown-src": "lowdown-src",
@@ -69,6 +85,7 @@
     },
     "root": {
       "inputs": {
+        "newNixpkgs": "newNixpkgs",
         "nix": "nix",
         "nixpkgs": [
           "nix",