How set java.io.tmpdir=/tmp for Jenkins slave?

[Ed of the Mountain]

Is there a way to set system property: java.io.tmpdir=/tmp for a slave?  I 
am launching the slave via SSH.  I assume if I can map 
java.io.tmpdir=/volume1/tmp 
my problem is solved.

My problem is I am running out of /tmp space with a Jenkins slave running 
on a Synology ARM based NAS which uses tmpfs or RAM as the /tmp resource.

Thanks in advance,

-Ed