fix build script for 6.0 to allow multiple options to be taken into account

IT-release-on-demand-build-nx60 job fails

https://qa2.nuxeo.org/jenkins/job/Deploy/job/IT-release-on-demand-build-nx60

For instance, enabling FINAL build (check the FINAL checkbox before launching the job) ends up with triggering the functional tests even if the SKIP_ITS option is enabled. (thanks Julien for spotting this!)

OPTIONS=()
if [ $FINAL = true ]; then
  OPTIONS+=(-f)
fi
if [ $SKIP_TESTS = true ]; then
  OPTIONS+=(--skipTests)
fi
if [ $SKIP_ITS = true ]; then
  OPTIONS+=(--skipITs)
fi