packages: use any nix for diff

microvms not necessarily need to have a nix, so this might be unbuilt

packages.nix

@@ -175,7 +175,7 @@ lib.attrsets.mapAttrs
         createSymlinks = name: ''
           if [[ -e old ]]; then
             echo System package diff:
-            ${discardStringCtx hostConfig.nix.package.outPath}/bin/nix --extra-experimental-features nix-command store diff-closures ./old ./current || true
+            nix --extra-experimental-features nix-command store diff-closures ./old ./current || true
           fi
 
           ln -sfT \$PWD/current /nix/var/nix/gcroots/microvm/${name}