nixos-rebuild: Fix ShellCheck issue

ShellCheck recommends
<https://github.com/koalaman/shellcheck/wiki/SC2174> splitting this into
`mkdir` + `chmod`, but based on the discussion with @bjornfor in #357447
it's probably better to keep the atomic directory creation and the
probable intent of the original code.

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -141,7 +141,7 @@ while [ "$#" -gt 0 ]; do
         fi
         if [ "$1" != system ]; then
             profile="/nix/var/nix/profiles/system-profiles/$1"
-            mkdir -p -m 0755 "$(dirname "$profile")"
+            (umask 022 && mkdir -p "$(dirname "$profile")")
         fi
         shift 1
         ;;