feat: Cache key uses repo.org only if USE_CONFIG_FROM_PULL_REQUEST is disabled.

[mhflink]

Change the cache key to repo.org if only global mergeable configuration can be used.

This logic applies only in a combination of

• USE_CONFIG_FROM_PULL_REQUEST = false
• USE_ORG_AS_DEFAULT_CONFIG = true
• USE_CONFIG_CACHE = true

Before this change, mergeable would already cache the org config for each repo, but would always use the org/repo cache key. Unfortunately if only global repository is used, this leads to a lot of cache misses if PRs are opened across many repositories within the organization.

In our case, the current behavior let to us being rate-limited by GitHub.