[tlbuild] Github Action fails for x86_

[Bruno Haible]

> Possibly related to the fact that RHEL 7 [1] and, with it, CentOS 7 [2]
> reached their end-of-life a couple of days ago.
> 
> [1] https://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux#Version_history_and_timeline
> [2] https://www.redhat.com/en/topics/linux/centos-linux-eol

A workaround has been posted here:
https://support.plesk.com/hc/en-us/articles/24575503258647-Plesk-or-system-updates-fail-on-CentOS-7-Could-not-resolve-host-mirrorlist-centos-org