Add Linux support

Linux support is already nearly there, I just need a bit of help getting it over the finish line from anyone who has more experience with Linux VST3.

The work involved is as follows:
* Inspect the VST3 bundle layout on Linux and update the [`getAssetsDirectory()`](https://github.com/elemaudio/srvb/blob/65e676d245e94a005bb9977e4cd4b52ec1a30ca8/native/PluginProcessor.cpp#L20) here for the `JUCE_LINUX` case
* Attach the native webview handle to the viewContainer for the `JUCE_LINUX` case [here](https://github.com/elemaudio/srvb/blob/65e676d245e94a005bb9977e4cd4b52ec1a30ca8/native/WebViewEditor.cpp#L61) and [here](https://github.com/elemaudio/srvb/blob/65e676d245e94a005bb9977e4cd4b52ec1a30ca8/native/WebViewEditor.h#L37)
* Re-enable CI running on linux [here](https://github.com/elemaudio/srvb/blob/65e676d245e94a005bb9977e4cd4b52ec1a30ca8/.github/workflows/main.yml#L16) and update the required dependency install step [here](https://github.com/elemaudio/srvb/blob/65e676d245e94a005bb9977e4cd4b52ec1a30ca8/.github/workflows/main.yml#L28) so that the builds are happy

I think that should be it, but it's very possible I'm missing something.