fix(n8n): resolve nix-update hash prefetch failure

- Remove --pure from nix-shell shebang to allow network access
- Add --flake --system x86_64-linux for proper flake evaluation
- Navigate to nixpkgs root before running nix-update
- Also bump version 2.17.5 -> 2.17.8

pkgs/n8n/default.nix

@@ -26,13 +26,13 @@
 in
   stdenv.mkDerivation (finalAttrs: {
     pname = "n8n";
-    version = "2.17.5";
+    version = "2.17.8";
 
     src = fetchFromGitHub {
       owner = "n8n-io";
       repo = "n8n";
       tag = "n8n@${finalAttrs.version}";
-      hash = "sha256-JwPrQOohXXeuUEcr5S+41ZElBJ3TxR3cgT45CjbzyR4=";
+      hash = "sha256-+Fr7zYca4+3R9J77TkHTGki8oz4FrE9MNsxOaDCF2GM=";
     };
 
     pnpmDeps = fetchPnpmDeps {