It's colon separated, using list type doesn't work on command line, and colon-separated makes a lot of sense for `$EASYBUILD_SOURCEPATH` (cfr. `$PATH`, `$MODULEPATH`, etc.). I'll update the help message.

Append to default list could be done, but gives less flexibility, since that means it would always be there... With 'overwrite', you can still explicitly include the default path too.