fix(ci): add 'test --cover' to preferred_envs

mix ci runs 'test --cover' as a space-separated alias, but the
preferred_envs config only had the dot-separated 'test.cover'.
In Elixir 1.20+, Mix warns (and eventually errors) when a task
runs in the wrong env. Adding the space-separated variant
silences the warning and ensures tests always run in :test env.

Author: gilbertwong96

mix.exs

@@ -172,7 +172,7 @@ defmodule AdoCli.MixProject do
   end
 
   def cli do
-    [preferred_envs: [test: :test, "test.cover": :test, "coveralls.json": :test]]
+    [preferred_envs: [test: :test, "test --cover": :test, "test.cover": :test, "coveralls.json": :test]]
   end
 
   defp escript_config do