Improve autoinstall script

Allow to use existing FairSoft and FairRoot installations.

requested review from @f.uhlig

assigned to @f.uhlig

Dear @f.uhlig, @v.friese, @p.-a.loizeau,

you have been identified as code owner of at least one file which was changed with this merge request.

Please check the changes and approve them or request changes.

added CodeOwners label

merged