[tlbuild] Github Action fails for x86_

[Bruno Haible]

> 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

In the context of GitHub actions, this workaround looks like this for me:

In the

      - uses: addnab/docker-run-action at v3
        with:
          image: centos:7

step's 'run' property, add the two lines

            : "The CentOS packages repository has moved on 2024-07-01."
            sed -i -e 's|^mirrorlist=|#mirrorlist=|' -e 's|^#baseurl=http://mirror\.centos\.org|baseurl=http://vault.centos.org|' /etc/yum.repos.d/CentOS-Base.repo

before the first use of 'yum'.

That's all that is needed for me. You may need to apply this workaround
also to the other files in the same directory:

  /etc/yum.repos.d/CentOS-CR.repo
  /etc/yum.repos.d/CentOS-Debuginfo.repo
  /etc/yum.repos.d/CentOS-Media.repo
  /etc/yum.repos.d/CentOS-Sources.repo
  /etc/yum.repos.d/CentOS-Vault.repo
  /etc/yum.repos.d/CentOS-fasttrack.repo
  /etc/yum.repos.d/CentOS-x86_64-kernel.repo

Bruno