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",

flake.nix

@@ -1,10 +1,13 @@
 {
   description = "A Nix-based continuous build system";
 
+  # FIXME: All the pinned versions of nix/nixpkgs have a broken foreman (yes,
+  # even 2.7.0's Nixpkgs pin).
+  inputs.newNixpkgs.url = "github:NixOS/nixpkgs/nixos-21.11";
   inputs.nixpkgs.follows = "nix/nixpkgs";
   inputs.nix.url = github:NixOS/nix/2.6.0;
 
-  outputs = { self, nixpkgs, nix }:
+  outputs = { self, newNixpkgs, nixpkgs, nix }:
     let
 
       version = "${builtins.readFile ./version.txt}.${builtins.substring 0 8 self.lastModifiedDate}.${self.shortRev or "DIRTY"}";
@@ -566,7 +569,9 @@
 
           checkInputs = [
             cacert
-            foreman
+            # FIXME: foreman is broken on all nix/nixpkgs pin, up to and
+            # including 2.7.0
+            newNixpkgs.legacyPackages.${final.system}.foreman
             glibcLocales
             netcat-openbsd
             openldap