Change drone builds for develop branch to use the server branch with the union pushdown feature

The union pushdown feature implemented in MDEV-25080 is available in the following server enterprise branch:

https://github.com/mariadb-corporation/MariaDBEnterprise/tree/10.6-MENT-1667

We need to update the drone builds for the MCS develop branch to use this server branch. This is required since the union pushdown feature involves updates to the storage engine API.

Once the 10.6-MENT-1667 branch is merged into the main enterprise server branch, we will have to update the MCS develop drone builds back to use the main enterprise server branch. I will let you know once this event happens.