The repositories input doesn't play nicly with github.repository

[Cube707]

When passing in a list of repos to scope the app's access to, github.repository can't be used (supprisingly).
Also building specific repos using ${{github.repository_owner}}/repo doesn't work.

This would be usefull when working across forks and / or reusing workflow snippets.

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• improve documentation to better reflect what input is expected
• filter the inputs to work with owners
• generally add github.repositor_name upstream into the github-context