frontend: fix a SQLAlchemy warning

    <stdin>:1: SAWarning: SELECT statement has a cartesian product between FROM
    element(s) "mock_chroot" and FROM element "copr_chroot".
    Apply join condition(s) between each element to resolve.

Author: FrostyX

frontend/coprs_frontend/coprs/logic/coprs_logic.py

@@ -1267,7 +1267,9 @@ def filter_outdated(cls, query):
 
         See https://docs.pagure.org/copr.copr/developer_documentation/eol-logic.html
         """
-        return (query.filter(models.CoprChroot.delete_after
+        return (query
+                .join(models.CoprChroot.mock_chroot)
+                .filter(models.CoprChroot.delete_after
                              >= datetime.datetime.now())
                      # Filter out chroots that are manually disabled by user
                      # (deleted, aka "unclicked", we never send e-mails there)