build: download_mirror fails due to a bad repo url

[Davlet Panech]

Brief Description
-----------------
EPEL source repo URLs appear to have changed and cause download errors on CENGN's in download_mirror script. This causes master branch builds to fail.

Severity
--------
Critical

Steps to Reproduce
------------------
Write down the steps to reproduce the issue

Expected Behavior
------------------
Download_mirror script works w/o errors. Master branch builds work w/o errors.

Actual Behavior
----------------
Download_mirror script fails. Master branch builds fail due to a missing download.

Reproducibility
---------------
Reproducible

System Configuration
--------------------
N/A

Branch/Pull Time/Commit
-----------------------
master/2021-06-30

Last Pass
---------
master/2021-6-28

Timestamp/Logs
--------------
(from jenkins job STX_download_mirror_layered #2040)

[...]
https://dl.fedoraproject.org/pub/epel/7/SRPMS/repodata/repomd.xml: [Errno 14] HTTPS Error 404 - Not Found
[...]
11:53:42 b5: Looking for linuxptp-2.0-2.el7.src.rpm
11:53:42 b5: --> run: yumdownloader -q -C -c /tmp/stx_mirror_PANBbs/yum.conf --releasever=7 --source linuxptp-2.0-2.el7
11:53:42 b5: Caching enabled but no local cache of /tmp/stx_mirror_PANBbs/cache/yum/x86_64/7/CENGN_StarlingX-C7.6.1810-atomic/f34a06abc50a0242beac01615029a633b03163bd1ed1ccb00640225ac1bced27-primary.sqlite.bz2 from CENGN_StarlingX-C7.6.1810-atomic
11:53:42 b5: Warning: linuxptp-2.0-2.el7.src.rpm not found
11:53:42 b5: Error: linuxptp-2.0-2.el7.src.rpm not found

Test Activity
-------------
N/A

Workaround
----------
Download the missing file manually before building from here: https://vault.centos.org/7.7.1908/os/Source/SPackages/linuxptp-2.0-2.el7.src.rpm

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/starlingx/tools/+/798949