How to override the default Nexus Temporary Directory

For Nexus 2.8.1+:

Edit ${NEXUS_HOME}/bin/jsw/conf/wrapper.conf, find the line containing wrapper.java.additional.2=-Djava.io.tmpdir=./tmp. Change the ./tmp value to your preferred absolute directory path that already exists and is RW capable by the Nexus process user. This should be a local disk location.

For Nexus 2.8.0 and earlier:

Edit ${NEXUS_HOME}/bin/jsw/conf/wrapper.conf, add another property wrapper.java.additional._n_=-Djava.io.tmpdir=/your/custom/tmp/dir where n is the next available number not already used.
Edit ${NEXUS_HOME}/conf/nexus.properties, add a property java.io.tmpdir=/absolute/path/to/your/custom/tmp/dir.