Add dotnet20 to --fixprefix to workaround the "rundll32.exe - This application could not be started." error

[whrvt]

This no longer appears if dotnet20 is in the prefix when wine updates the prefix (e.g. when a new version is installed)

Installing dotnet20 adds about a minute to the --fixprefix command, but is probably worth it in order to reduce confusion.

P.S. thank github for adding that important newline