replace flake-utils input with nix-systems

We're only using `eachDefaultSystem` from flake-utils so pulling it in is
a bit overkill; using the underlying nix-systems dependency directly brings
us closer to a "stock" flake format without adding unnecessary burden.

flake.lock

@@ -16,24 +16,6 @@
         "type": "github"
       }
     },
-    "flake-utils": {
-      "inputs": {
-        "systems": "systems"
-      },
-      "locked": {
-        "lastModified": 1685518550,
-        "narHash": "sha256-o2d0KcvaXzTrPRIo0kOLV0/QXHhDQ5DTi+OxcjO8xqY=",
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "rev": "a1720a10a6cfe8234c0e93907ffe81be440f4cef",
-        "type": "github"
-      },
-      "original": {
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "type": "github"
-      }
-    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1686488075,
@@ -52,11 +34,11 @@
     },
     "nixpkgs_2": {
       "locked": {
-        "lastModified": 1680945546,
-        "narHash": "sha256-8FuaH5t/aVi/pR1XxnF0qi4WwMYC+YxlfdsA0V+TEuQ=",
+        "lastModified": 1688590700,
+        "narHash": "sha256-ZF055rIUP89cVwiLpG5xkJzx00gEuuGFF60Bs/LM3wc=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "d9f759f2ea8d265d974a6e1259bd510ac5844c5d",
+        "rev": "f292b4964cb71f9dfbbd30dc9f511d6165cd109b",
         "type": "github"
       },
       "original": {
@@ -69,8 +51,8 @@
     "root": {
       "inputs": {
         "flake-compat": "flake-compat",
-        "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs",
+        "systems": "systems",
         "treefmt": "treefmt"
       }
     },
@@ -84,9 +66,8 @@
         "type": "github"
       },
       "original": {
-        "owner": "nix-systems",
-        "repo": "default",
-        "type": "github"
+        "id": "systems",
+        "type": "indirect"
       }
     },
     "treefmt": {