chore: add dedicated nixpkgs input to flake

This ensures that `nix flake update` doesn't create surprises on
different systems.

flake.lock

@@ -2,9 +2,7 @@
   "nodes": {
     "fenix": {
       "inputs": {
-        "nixpkgs": [
-          "nixpkgs"
-        ],
+        "nixpkgs": ["nixpkgs"],
         "rust-analyzer-src": "rust-analyzer-src"
       },
       "locked": {
@@ -41,9 +39,7 @@
     },
     "naersk": {
       "inputs": {
-        "nixpkgs": [
-          "nixpkgs"
-        ]
+        "nixpkgs": ["nixpkgs"]
       },
       "locked": {
         "lastModified": 1698420672,
@@ -61,16 +57,18 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1698846319,
-        "narHash": "sha256-4jyW/dqFBVpWFnhl0nvP6EN4lP7/ZqPxYRjl6var0Oc=",
+        "lastModified": 1712168706,
+        "narHash": "sha256-XP24tOobf6GGElMd0ux90FEBalUtw6NkBSVh/RlA6ik=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "34bdaaf1f0b7fb6d9091472edc968ff10a8c2857",
+        "rev": "1487bdea619e4a7a53a4590c475deabb5a9d1bfb",
         "type": "github"
       },
       "original": {
-        "id": "nixpkgs",
-        "type": "indirect"
+        "owner": "NixOS",
+        "ref": "nixos-23.11",
+        "repo": "nixpkgs",
+        "type": "github"
       }
     },
     "root": {

flake.nix

@@ -1,5 +1,6 @@
 {
   inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
     flake-utils.url = "github:numtide/flake-utils";
 
     # for quicker rust builds