Fix broken CTA containerised installation

The CTA containerised installation is failing (https://eoscta.docs.cern.ch/gitlab/runner/).

This is due to a broken symlink in the ctafrontend container, cause by a directory change in the commit bfc8eec2:

https://gitlab.cern.ch/cta/CTA/-/blob/81aee933/continuousintegration/docker/ctafrontend/cc7/opt/run/bin/mkSymlinks.sh#L28

xroot_plugins/cta-frontend-xrootd.conf → continuousintegration/docker/ctafrontend/cc7/etc/cta/cta-frontend-xrootd.conf

[root@ctafrontend cta]# tail cta-frontend-xrootd.log
220722 10:30:08 209 XrdConfig: Unable to open config file /etc/cta/cta-frontend-xrootd.conf; no such file or directory
220722 10:30:08 209 XrdConfig: Unable to create home directory /home/cta/cta; read-only file system
------ xrootd cta@ctafrontend.cta.svc.cluster.local:-1 initialization failed.
220722 14:00:06 200 Starting on Linux 3.10.0-1160.71.1.el7.x86_64
Copr.  2004-2012 Stanford University, xrd version v4.12.6
++++++ xrootd cta@ctafrontend.cta.svc.cluster.local initialization started.
Config using configuration file /etc/cta/cta-frontend-xrootd.conf
220722 14:00:06 200 XrdConfig: Unable to open config file /etc/cta/cta-frontend-xrootd.conf; no such file or directory
220722 14:00:06 200 XrdConfig: Unable to create home directory /home/cta/cta; read-only file system
------ xrootd cta@ctafrontend.cta.svc.cluster.local:-1 initialization failed.