Reset the state of StubENV's @env_already_stubbed after each spec run

In certain combination of tests, @env_already_stubbed could be set
to `true` even though it never ran for that specific test.

spec/support/stub_env.rb

@@ -5,3 +5,11 @@ module StubENV
     allow(ENV).to receive(:[]).with(key).and_return(value)
   end
 end
+
+# It's possible that the state of the class variables are not reset across
+# test runs.
+RSpec.configure do |config|
+  config.after(:each) do
+    @env_already_stubbed = nil
+  end
+end