Make the script for downloading missing MAR files smarter

We should only need to download the MAR files we need for the particular context the respective script is invoked in. E.g. if it is invoked when creating the new MAR files after macOS code signing there is no need to download the non-macOS MAR files as well but just the needed macOS ones. However, if we want to create incrementals for all platoforms later on (for whatever reason) the script should download the missing MAR files for the other platforms we support.