Stub ENV in the LDAP task spec file

Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/tasks/gitlab/ldap_rake_spec.rb

@@ -4,7 +4,7 @@ describe 'gitlab:ldap:rename_provider rake task' do
   it 'completes without error' do
     Rake.application.rake_require 'tasks/gitlab/ldap'
     stub_warn_user_is_not_gitlab
-    ENV['force'] = 'yes'
+    stub_env('force', 'yes')
 
     create(:identity) # Necessary to prevent `exit 1` from the task.