refactor(flake): use flake-parts

This makes it easier to split up the flake in the future.

flake.lock

@@ -1,12 +1,32 @@
 {
   "nodes": {
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1727826117,
+        "narHash": "sha256-K5ZLCyfO/Zj9mPFldf3iwS6oZStJcU4tSpiXTMYaaL0=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "3d04084d54bedc3d6b8b736c70ef449225c361b1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1727348695,
-        "narHash": "sha256-J+PeFKSDV+pHL7ukkfpVzCOO7mBSrrpJ3svwBFABbhI=",
+        "lastModified": 1728492678,
+        "narHash": "sha256-9UTxR8eukdg+XZeHgxW5hQA9fIKHsKCdOIUycTryeVw=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "1925c603f17fc89f4c8f6bf6f631a802ad85d784",
+        "rev": "5633bcff0c6162b9e4b5f1264264611e950c8ec7",
         "type": "github"
       },
       "original": {
@@ -17,6 +37,7 @@
     },
     "root": {
       "inputs": {
+        "flake-parts": "flake-parts",
         "nixpkgs": "nixpkgs",
         "systems": "systems"
       }