Add clean support within the prep.sh script

The prep.sh script that was recently added as part of the [Tarball restructuring work](https://github.com/dotnet/source-build/issues/2143) should support an option to cleanup the artifacts it has downloaded as part of a previous run. If the script detects a previous artifact, it will not re-download it. There are situations in which you would like to cleanup the previous artifacts and possibly redownload them (e.g. you have a corrupt version of the artifact).