Makefile.rules: Disable planex-cache by default
Makefile.rules: Disable planex-cache by default

There is an undiagnosed race between createrepo and planex-cache
in concurrent builds.   Ordinary mock is not affected.

Signed-off-by: Euan Harris <c3b6e83069c8e9e3af49a38fec6026be89559638@citrix.com>

djs55/planex,djs55/planex,djs55/planex