recipient_for_emails: Fix type error

Fixes
[
  File "/srv/zulip/zerver/lib/actions.py", line 605, in recipient_for_emails
    if not (normalized_emails & admin_realm_admin_emails or normalized_emails & settings.CROSS_REALM_BOT_EMAILS):
TypeError: unsupported operand type(s) for &: 'set' and 'list'

(imported from commit f39a95dad7b3207e9188fc03926cd116061ef3f3)

zerver/lib/actions.py

@@ -600,7 +600,7 @@ def recipient_for_emails(emails, not_forged_mirror_message,
     if len(realm_domains) == 2:
         # I'm assuming that cross-realm PMs with the "admin realm" are rare, and therefore can be slower
         admin_realm = Realm.objects.get(domain=settings.ADMIN_DOMAIN)
-        admin_realm_admin_emails = [u.email for u in admin_realm.get_admin_users()]
+        admin_realm_admin_emails = {u.email for u in admin_realm.get_admin_users()}
         # We allow settings.CROSS_REALM_BOT_EMAILS for the hardcoded emails for the feedback and notification bots
         if not (normalized_emails & admin_realm_admin_emails or normalized_emails & settings.CROSS_REALM_BOT_EMAILS):
             raise ValidationError("You can't send private messages outside of your organization.")