Teach test_sync to use a diff EPEL mirror

[ggainey]

Version
main

Describe the bug
Nightly CI fails regularly (3+ times per week) with the following error (which can be cleared by re-running)"

usr/local/lib/python3.8/site-packages/pulp_rpm/tests/functional/api/test_sync.py:132: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
usr/local/lib/python3.8/site-packages/pulp_rpm/tests/functional/conftest.py:224: in _init_and_sync
    task = monitor_task(sync_response.task)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

task_href = '/pulp/api/v3/tasks/ef5d5c5b-9098-4732-8141-a452013988c2/'

    def _monitor_task(task_href):
        while True:
            try:
                task = tasks_api_client.read(task_href)
            except ApiException as e:
                if pulp_domain_enabled and e.status == 404:
                    # Task's domain has been deleted, nothing to show anymore
                    return {}
                raise e
    
            if task.state in ["completed", "failed", "canceled"]:
                break
            sleep(SLEEP_TIME)
    
        if task.state != "completed":
>           raise PulpTaskError(task=task)
E           pulp_smash.pulp3.bindings.PulpTaskError: (PulpTaskError(...), 
"Pulp task failed (
  A file located at the url http://ftp.uni-bayreuth.de/linux/fedora-epel/8/Modular/x86_64/repodata/7d5fdce5a87c14cb3b418eaee930a606f4ef1e1ae1da18faaabce1fed6974425-other.xml.gz 
  failed validation due to checksum. 
  Expected '7d5fdce5a87c14cb3b418eaee930a606f4ef1e1ae1da18faaabce1fed6974425', 
  Actual '0938e4396160d828c4576cf7b1bba5793eed20b8c81947b1a71c94fbbe80d118')")

It is always ftp.uni-bayreuth.de; can we find a more-reliable mirror to use for this test?