tmpfiles: keep directory specifiers rootless

%t and the other directory specifiers are parsed as normal paths.
They are prefixed by --root= later, so the specifier itself must stay
rootless.

Reproducer:
  tmp=$(mktemp -d /tmp/tmpfiles-root.XXXXXX)
  conf=$(mktemp /tmp/tmpfiles-conf.XXXXXX)
  printf 'd %%t/tmpfiles-repro 0755 - - -\n' >"$conf"
  systemd-tmpfiles --root="$tmp" --create --dry-run "$conf"

Before:
  Would create directory $tmp/$tmp/run/tmpfiles-repro

Follow-up for de61a04b18.

src/tmpfiles/tmpfiles.c

@@ -322,16 +322,7 @@ static int specifier_directory(
         if (r < 0)
                 return r;
 
-        if (arg_root) {
-                _cleanup_free_ char *j = NULL;
-
-                j = path_join(arg_root, p);
-                if (!j)
-                        return -ENOMEM;
-
-                *ret = TAKE_PTR(j);
-        } else
-                *ret = TAKE_PTR(p);
+        *ret = TAKE_PTR(p);
 
         return 0;
 }