Update and fix nix flake (#341)

Since nix tries to update inputs on newer versions out of, I suppose,
some introduced incompatibility, `nix build` fails. And nixpkgs from
2023 sounds rather obsolete at this point, so update was kinda due I
think.
On newer versions of nixpkgs however buildGoModule has changed and
requires one of its arguments to be called vendorHash and not
vendorSha256, this patch changes that as well.

Fixes #297, closes #314, closes #307

flake.lock

@@ -2,17 +2,18 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1703013332,
-        "narHash": "sha256-+tFNwMvlXLbJZXiMHqYq77z/RfmpfpiI3yjL6o/Zo9M=",
+        "lastModified": 1740367490,
+        "narHash": "sha256-WGaHVAjcrv+Cun7zPlI41SerRtfknGQap281+AakSAw=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "54aac082a4d9bb5bbc5c4e899603abfb76a3f6d6",
+        "rev": "0196c0175e9191c474c26ab5548db27ef5d34b05",
         "type": "github"
       },
       "original": {
-        "id": "nixpkgs",
+        "owner": "NixOS",
         "ref": "nixos-unstable",
-        "type": "indirect"
+        "repo": "nixpkgs",
+        "type": "github"
       }
     },
     "root": {