Revert "bump nixpkgs"

The last nixpkgs update introduced a regression in Prometheus 2.49.0 that
broke monitoring of zkapissuer in staging.  A hotfix to Prometheus (2.49.1)
was released an hour after the broken version, but NixOS 23.11 still has not
received that three days later.

This reverts commit 580362b6.

nixpkgs.json

 {
   "name": "source",
-  "url": "https://releases.nixos.org/nixos/23.11/nixos-23.11.3920.25e3d4c0d359/nixexprs.tar.xz",
-  "sha256": "1wx3qxnz0pm3ld7k6gfvlxs7cv8dvj26917c3136ihvg7znfj1i8"
+  "url": "https://releases.nixos.org/nixos/23.11/nixos-23.11.2962.b8dd8be3c790/nixexprs.tar.xz",
+  "sha256": "053dwx7smszz3a0khbm6zvf9pvy5xz1ifdk9jx6gz5cmwjab0gmp"
 }
\ No newline at end of file