Ignore the generated postbuild.groovy script

The Jenkins job at http://jenkins.imagej.net/job/ImageJ1-releases needs to
write out a postbuild.groovy script (written out during the run of
release.sh by slightly modifying make-jenkins-links.groovy) because the
Groovy PostBuild has no access to the variables that the main Build Step
has, but needs to know about them to attach the appropriate badges.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>