chore: modernize Astro Nix development shell

flake.lock

@@ -2,16 +2,18 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1728018373,
-        "narHash": "sha256-NOiTvBbRLIOe5F6RbHaAh6++BNjsb149fGZd1T4+KBg=",
-        "rev": "bc947f541ae55e999ffdb4013441347d83b00feb",
-        "revCount": 688563,
-        "type": "tarball",
-        "url": "https://api.flakehub.com/f/pinned/NixOS/nixpkgs/0.1.688563%2Brev-bc947f541ae55e999ffdb4013441347d83b00feb/01925b66-3b70-71bd-b277-924a6c208ba7/source.tar.gz"
+        "lastModified": 1785090369,
+        "narHash": "sha256-m0pDuRJG7EDo9ri+4Ksu83VsI+PlxNC9lNBfydejce4=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "624af665418d3c65d544145b4d34ad696439570e",
+        "type": "github"
       },
       "original": {
-        "type": "tarball",
-        "url": "https://flakehub.com/f/NixOS/nixpkgs/0.1.%2A.tar.gz"
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
       }
     },
     "root": {