Fix failing CI/CD add/remove variable test
The selector name was wrong, causing the QA smoke test to fail.
The QA selector test likely passed because app/assets/javascripts/ci_variable_list/components/ci_variable_modal.vue
(https://gitlab.com/gitlab-org/gitlab/blob/6a9a5d779e2b9ce4a031babc108efaa906a07896/app/assets/javascripts/ci_variable_list/components/ci_variable_modal.vue#L174) has the right selector, but the newly-introduced app/assets/javascripts/ci_variable_list/components/ci_key_field.vue
(which is behind a feature flag) did not.
Closes #214910 (closed)
Edited by Stan Hu