Add .runner_system_id to .gitignore

What does this merge request do and why?

Seems that when the runner is used, a .runner_system_id file is created. Let's add it to .gitignore.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• [-] Tests added for new functionality. If not, please raise an issue to follow-up.
• [-] Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.
• [-] Add the ~highlight label if this MR should be included in the CHANGELOG.md.