Add capability of testing canary CI runs by apache/airflow PR

[potiuk]

It's been impossible to test canary runs as they are only run when you push to main or one of the v2-*-test branch in apache/airlfow repo. This PR adds capability to run special PR with canary label to test canary run workflow in a PR from committer run in the apache/airflow repo.

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[hussein-awala]

LGTM

[potiuk]

Also that's a nice way to refresh constraints and cache it turns out. I have sometimes problems with main canary build to succeed because subsequent merges to main will cancel running canary builds, so if there are merges happening every 20/30 minutes it prevents the cache from being refreshed and constraints from being updated, But this one should nicely allow doing that by opening a canary PR :)