community/mumble: Separate lib and plugin dir

The path explicitly set for MUMBLE_INSTALL_PLUGINDIR was identical to MUMBLE_INSTALL_LIBDIR causing packaged libraries (CELT + overlay) to end up in what Mumble considers the plugin directory, which will produce warning messages every time Mumble is started. See e.g. https://github.com/mumble-voip/mumble/issues/5494

Based on my judgement, explicitly setting CMAKE_INSTALL_LIBDIR is essentially also a no-op but since that is not directly relevant to the observed issue, I left it unchanged.

Note that by not explicitly setting the plugin directory, it will default to a directory "plugins" inside MUMBLE_INSTALL_LIBDIR.