Replace deprecated nodePackages.pnpm with pnpm

nodePackages has been removed from nixpkgs. Use the standalone pnpm
package instead.

modules/lamassu-lnbits.nix

@@ -272,7 +272,7 @@ in
       wants = [ "network-online.target" ];
 
       path = with pkgs; [
-        nodejs_22 nodePackages.pnpm python3 git coreutils bash util-linux
+        nodejs_22 pnpm python3 git coreutils bash util-linux
         stdenv.cc gnumake pkg-config binutils expat
       ];
 
@@ -406,7 +406,7 @@ in
     # Helper tools
     environment.systemPackages = with pkgs; [
       nodejs_22
-      nodePackages.pnpm
+      pnpm
       postgresql
       (writeShellScriptBin "lamassu-register-user" ''
         DB_PASSWORD=$(cat ${secretsDir}/lamassu-db-password)