Use lb-run --siteroot in Ganga input sandbox script

This makes the run script in the Ganga input sandbox independent from a fully fledged LHCb environment: the only requirement is lb-run in the PATH (version >= 0.3.5).

By default it uses the released software from directory specified in $MYSITEROOT at runtime, or, if not set, the $MISITEROOT value when the input sandbox was created (or /cvmfs/lhcb.cern.ch/lib if it was not set at build time).