Try using workflow runs for the webhooks

[armenzg]

We're currently using workflow jobs:

@asottile-sentry mentions that it is possible to use workflow runs for this app. This was not originally available when the prototype was written.

This could potentially save these extra two calls:
https://github.com/getsentry/github-actions-app/blob/09c1a16e717ad750dc3b249ebb214356492c949b/src/github_sdk.py#L43-L44

That could potentially save more than 70% of the execution time:

[armenzg]

This may be necessary to avoid having to change permissions for external orgs.