services: cuirass: Cache defaults to /var/cache/cuirass.

The previous default value depended on the 'HOME' environment variable,
which happened to be unset.  Thus, /.cache was being used.

* gnu/services/cuirass.scm (<cuirass-configuration>)[cache-directory]:
Change default value to "/var/cache/cuirass".
(cuirass-shepherd-service): Always pass "--cache-directory".
(cuirass-activation): New procedure.
(cuirass-service-type): Use it as an extension to
ACTIVATION-SERVICE-TYPE.
* doc/guix.texi (Continuous Integration): Adjust accordingly.