Virtualbox-extpack installation process breaks after update

[guiweber]

We need to update the filename option since it contains the version number. The installer needs to have the right file name to install properly.

[guiweber]

Note: I checked if this could be fixed upstream in Just-Install by using the {{.version}} placeholder but unfortunately the variable part of the file name is more than just the version number.

Also, the updater already loads the entire registry (not just the urls) and we can easily rewrite the options by using the global var registry.packages[app], so It looks like it will be easier to fix in the updater