Fix use of strace

version.nix

@@ -20,11 +20,11 @@ let
     [[ $(id -un) == root ]] || {
         exec sudo -H $0 "$@"
     }
-    set -o xtrace
     latest=$(${pkgs.git}/bin/git ls-remote ${repo} ${ref}|cut -f1)
     if [[ "${hash}" == "$latest" ]]; then
       echo Already up to date.
     else
+      set -o xtrace
       nix-channel --update liucs
       nixos-rebuild switch
     fi