cache: Add PLANEX_CACHEDIR environment variable
cache: Add PLANEX_CACHEDIR environment variable

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

euanh/planex-cleanhistory,euanh/planex-cleanhistory,simonjbeaumont/planex,euanh/planex-cleanhistory,djs55/planex,djs55/planex,djs55/planex,simonjbeaumont/planex,simonjbeaumont/planex