Disallow shim execution mode if executor supports native steps execution

If a CI job was specified as steps and the target executor supports native steps execution, force native steps execution. In other words, do not allow execution of the job via shim mode if the executor supports native steps execution.

Related to Disallow shim execution mode if executor suppor... (step-runner#297 - closed)

Closes Disallow shim execution mode if executor suppor... (step-runner#297 - closed)