Mariadb revert
Revert back to mysql 5.7 in order to avoid breaking centos7-gcc{8,9} builds. This is related to the issue of making mariadb relocatable, as mariadb_config is a binary which remembers the path to where the build took place. Replacing it with a symlink to mysql_config triggered a different bug where infinite recursion would break the build nodes.
Edited by Richard Bachmann