fix(n8n): use stable tag target to get actual version

The previous jq filter grabbed the first non-stable release by creation date,
which incorrectly returned 1.123.38 instead of the latest stable 2.18.5.

Now query the 'stable' tag directly and extract version from its
target_commitish (e.g., 'release/2.18.5' -> '2.18.5'), ensuring we always
get the actual latest stable version.

Also bump version from 2.17.8 to 2.18.5 with updated hashes.

pkgs/n8n/default.nix

@@ -26,20 +26,20 @@
 in
   stdenv.mkDerivation (finalAttrs: {
     pname = "n8n";
-    version = "2.17.8";
+    version = "2.18.5";
 
     src = fetchFromGitHub {
       owner = "n8n-io";
       repo = "n8n";
       tag = "n8n@${finalAttrs.version}";
-      hash = "sha256-+Fr7zYca4+3R9J77TkHTGki8oz4FrE9MNsxOaDCF2GM=";
+      hash = "sha256-ws0DXGQFR+z3nVyd4Yn9pIM7yh+H6GnuCRSLxgvtPxo=";
     };
 
     pnpmDeps = fetchPnpmDeps {
       inherit (finalAttrs) pname version src;
       pnpm = pnpm_10;
       fetcherVersion = 3;
-      hash = "sha256-MBSxAsZXCaxwQpstJVxOOCIAE+0RqwlIrgXtE/hiTJM=";
+      hash = "sha256-Ajgne0neNm6HgMK6z3jnEkUJJxVOTgzjpSaMaJgIndQ=";
     };
 
     nativeBuildInputs =