nixos-rebuild-ng: use hash token in ControlPath

Previously used token %n refers to the hostname
which can be `too long for Unix domain socket`

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/process.py

@@ -19,7 +19,7 @@ SSH_DEFAULT_OPTS: Final = [
     "-o",
     "ControlMaster=auto",
     "-o",
-    f"ControlPath={tmpdir.TMPDIR_PATH / 'ssh-%n'}",
+    f"ControlPath={tmpdir.TMPDIR_PATH / 'ssh-%C'}",
     "-o",
     "ControlPersist=60",
 ]