nixos/ollama: replace gnu parallel with xargs for paralellism

Switch the model pulling mechanism from pkgs.parallel to standard xargs paired with nproc. This removes GNU Parallel (Perl runtime dependency) from system closure.

Dynamically limit parallel execution threads to the system core count using nproc.

nixos/modules/services/misc/ollama.nix

@@ -306,16 +306,18 @@ in
       script =
         let
           binaryInputs = lib.mapAttrs (_: lib.getExe) {
-            parallel = pkgs.parallel;
             awk = pkgs.gawk;
             sed = pkgs.gnused;
           };
+
           inherit (binaryInputs)
-            parallel
             awk
             sed
             ;
 
+          nproc = lib.getExe' pkgs.coreutils "nproc";
+          xargs = lib.getExe' pkgs.findutils "xargs";
+
           declaredModelsRegex = lib.pipe cfg.loadModels [
             (map lib.escapeRegex)
             (lib.concatStringsSep "|")
@@ -344,7 +346,7 @@ in
             fi
           ''}
 
-          '${parallel}' --tag '${ollama}' pull ::: ${lib.escapeShellArgs cfg.loadModels}
+          printf "%s\0" ${lib.escapeShellArgs cfg.loadModels} | '${xargs}' -0 -r -n 1 -P "$('${nproc}')" '${ollama}' pull
         '';
     };