Use commenter's repository permissions instead of author association.

[jastang]

Description of your changes

Determine appropriate access for comment-based workflow triggers based on the commenter's repository-level permission, rather than author association (which is a non-intuitive derivation based on membership in the GitHub organization in addition to repository-level permissions).

This aims to ensure trusted actors are able to run their e2e workflows in repositories they maintain, without necessarily requiring membership in the crossplane-contrib organization.

I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Tested in a personal fork of the repository. This is the first standalone repository to try adapting this pattern -- this will need to be updated in multiple other repositories including the workflows-only repo that others inherit from.
[contribution process]: https://git.io/fj2m9

[jastang]

If we want to be fancy we can have this reply to the comment if the user does not have sufficient permissions but that can be outside the scope of this change.