wsrep-new-cluster and wsrep-start-position in wrong directory with wrong selinux permissions

After moving the systemd service to using environment files instead of `systemctl set-environment` in 11.6 (MDEV-19210), they (wsrep-new-cluster and wsrep-start-position) are located in /var/lib/mysql along with the socket file in Fedora/RHEL-based distros. This causes them to have incorrect selinux permissions and therefore be not readable by systemd.

A solution is to generate these files in the run directory, instead, which already has correct permissions. Dissociating these files and the socket in CMake configs can also prove useful for other things.

Another alternative is to move the socket file, too. I confirmed that this way the package builds fine and all the tests pass. Unfortunately, reverse dependencies may not respond so well to this.