fix: use nixos-25.11 with apple-sdk_15 for darwin support (fixes #1)

- Switch to nixos-25.11 for modern apple-sdk packages
- Use apple-sdk_15 which includes SecTrustCopyCertificateChain (macOS 12+)
- Required for Go 1.25 crypto/x509 compatibility on darwin

flake.lock

@@ -2,16 +2,16 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1765687488,
-        "narHash": "sha256-7YAJ6xgBAQ/Nr+7MI13Tui1ULflgAdKh63m1tfYV7+M=",
+        "lastModified": 1766201043,
+        "narHash": "sha256-eplAP+rorKKd0gNjV3rA6+0WMzb1X1i16F5m5pASnjA=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "d02bcc33948ca19b0aaa0213fe987ceec1f4ebe1",
+        "rev": "b3aad468604d3e488d627c0b43984eb60e75e782",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
-        "ref": "nixos-25.05",
+        "ref": "nixos-25.11",
         "repo": "nixpkgs",
         "type": "github"
       }