Stop using Steinberg VST2 library to fix CI

[cpdt]

Steinberg are in the process of winding down VST2, and seem to have removed some necessary parts from their download, which are used for the CI build. Various other projects which include the SDK in their source have been hit with DMCA takedowns, so it looks like we'll need to move to something else to provide the VST2 library.

LMMS has the VeSTige header which looks like the best option. It is GPL licensed, however, which might cause a problem with our MIT license. Some research will be necessary to see if we're allowed to use a similar approach to now where the header isn't included in our source, but must be downloaded separately when building the VST backend.