security/amavisd-milter: resolve runtime bug with temp directory handling

• Use ${AMAVISDIR}/tmp as working directory to resolve runtime bug: Failure: Suspicious temporary directory name '/var/run/amavis/tmp/afcokTMBJlIs'

PR: 204787
Submitted by: Geoffroy Desvernay <dgeo@centrale-marseille.fr>
Approved by: Petr Rehor <rx@rx.cz> (maintainer)
MFH: 2015Q4