define JENKINS_HOME before using it

all those "log gc" options never really worked because JENKINS_HOME was
not defined and so leaded only to messages like
    OpenJDK 64-Bit Server VM warning: Cannot open file /gc-2017-12-11_17-04-12.log due to Permission denied
during startup.

While on it also move them into a subdirectory.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>