pipeline_variables_minimum_override_role should not break the Trigger Pipeline API

Problem to solve

When you Restrict pipeline variables to No one and then trigger a pipeline via the API, you receive a Insufficient permissions to set pipeline variables error.

That's because of the TRIGGER_PAYLOAD variable that is always added in https://gitlab.com/gitlab-org/gitlab/-/blob/b5de9978de0b357aa91319efe5e76d0ec1ccb983/app/services/ci/pipeline_trigger_service.rb#L89. This variable is an implementation detail and not a user explicitly setting a pipeline variable.

Proposal

Triggering a pipeline via the API should work regardless of the Restrict pipeline variables setting when the user does not define any variables in the API call. However, the setting should still prevent users from specifically passing variables.

There is precedent for adding an exception: #513148 (closed)