chore(nix): modernize flake to use buildGoModule

- Remove flake-utils dependency (use local forAllSystems helper)
- Remove gomod2nix dependency (use native buildGoModule)
- Add overlay export for easy consumption
- Update nixpkgs to latest unstable
- Disable tests in nix build (require network access)

flake.lock

@@ -1,35 +1,12 @@
 {
   "nodes": {
-    "gomod2nix": {
-      "inputs": {
-        "nixpkgs": [
-          "nixpkgs"
-        ],
-        "utils": [
-          "utils"
-        ]
-      },
-      "locked": {
-        "lastModified": 1677459247,
-        "narHash": "sha256-JbakfAiPYmCCV224yAMq/XO0udN5coWv/oazblMKdoY=",
-        "owner": "tweag",
-        "repo": "gomod2nix",
-        "rev": "3cbf3a51fe32e2f57af4c52744e7228bab22983d",
-        "type": "github"
-      },
-      "original": {
-        "owner": "tweag",
-        "repo": "gomod2nix",
-        "type": "github"
-      }
-    },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1693844670,
-        "narHash": "sha256-t69F2nBB8DNQUWHD809oJZJVE+23XBrth4QZuVd6IE0=",
+        "lastModified": 1767364772,
+        "narHash": "sha256-fFUnEYMla8b7UKjijLnMe+oVFOz6HjijGGNS1l7dYaQ=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "3c15feef7770eb5500a4b8792623e2d6f598c9c1",
+        "rev": "16c7794d0a28b5a37904d55bcca36003b9109aaa",
         "type": "github"
       },
       "original": {
@@ -41,42 +18,7 @@
     },
     "root": {
       "inputs": {
-        "gomod2nix": "gomod2nix",
-        "nixpkgs": "nixpkgs",
-        "utils": "utils"
-      }
-    },
-    "systems": {
-      "locked": {
-        "lastModified": 1681028828,
-        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
-        "owner": "nix-systems",
-        "repo": "default",
-        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-systems",
-        "repo": "default",
-        "type": "github"
-      }
-    },
-    "utils": {
-      "inputs": {
-        "systems": "systems"
-      },
-      "locked": {
-        "lastModified": 1692799911,
-        "narHash": "sha256-3eihraek4qL744EvQXsK1Ha6C3CR7nnT8X2qWap4RNk=",
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "rev": "f9e7cf818399d17d347f847525c5a5a8032e4e44",
-        "type": "github"
-      },
-      "original": {
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "type": "github"
+        "nixpkgs": "nixpkgs"
       }
     }
   },