improve liminix-rebuild test

* make it executable
* improve robustness
* do't hardcode services.default (why did it do this?)

tests/min-copy-closure/test-liminix-rebuild.sh

@@ -1,10 +1,11 @@
+#!/usr/bin/env nix-shell
+#! nix-shell -v -i bash -p expect socat
+
 # This is a test for liminix-rebuild. It's not a CI test because
 # liminix-rebuild calls nix-build so won't run inside a derivation,
 # meaning you have to remember to run it manually when changing
 # liminix-rebuild
 
-# nix-shell -p  expect socat --run "sh ./tests/min-copy-closure/test-liminix-rebuild.sh "
-
 . tests/test-helpers.sh
 
 set -e