Auto merge of #126352 - cuviper:centos7-vault, r=Kobzol

ci: Update centos:7 to use vault repos

CentOS 7 is going EOL on June 30, after which its package repos will no
longer exist on the regular mirrors. We'll still be able to access
packages from the vault server though, and can start doing so now. This
affects `dist-i686-linux` and `dist-x86_64-linux`.

I also removed `epel-release` because we were only using that for its
`cmake3`, but we've been building our own version for a while.

try-job: dist-i686-linux
try-job: dist-x86_64-linux

Author: bors