Use pull_request.user, not actor to determine PR user (#15504)

In most cases these are the same -- the one exception is when
(re)opening an issue, in which case the actor is going to be someone
with commit rights to a repo, and we don't want the mere act of
re-opening to cause a PR to run on self-hosted infrastructure as that
would be surprising (and potentially unsafe)

.github/workflows/ci.yml

@@ -122,7 +122,7 @@ jobs:
             "xinbinhuang",
             "yuqian",
             "eladkal"
-          ]'), github.actor)
+          ]'), github.event.pull_request.user.login)
         ) && github.repository == 'apache/airflow'
       ) && 'self-hosted' || 'ubuntu-20.04' }}
     env: