maintainers: add happy-river

maintainers/maintainer-list.nix

···
     githubId = 5317234;
     name = "Raphael Megzari";
   };
+  happy-river = {
+    email = "[e-mail]";
+    github = "happy-river";
+    githubId = 54728477;
+    name = "Happy River";
+  };
   haslersn = {
     email = "[e-mail]";
     github = "haslersn";