add repository_name to expr_sema.go

[mm-chia]

This property is undocumented in the official Github documentation, however it is perfectly valid and actionlint errors out when it's being used.

Attempts to fix https://github.com/rhysd/actionlint/issues/529

[mm-chia]

Hi there, Is any chance on getting this merged soon? thanks!

[rhysd]

This is not listed because the repository_name prop is undocumented. Do you have any pointer which describes that this is an official property?

https://docs.github.com/en/actions/reference/workflows-and-actions/contexts#github-context

[rhysd]

Closing this because #529 was closed.