Can job 'run_as' be assigned to users/principals who actually run it? instead of always a fixed creator/user/pricipal?

When a job is run, I would like to see in the job setting "run_as" the name of the actual user/principal who runs it.

Currently, "run_as" is set to a fixed user/principal. If someone else runs this job (either manually or using APIs), "run_as" is still showed as the fixed user/principal, and not the actual user/principal who runs it.

Am I missing something or this feature is actually not available?