Adapt build system for in-tree JuliaSyntax
These are the simplest possible adaptions to create the vendored
Base.JuliaSyntax from an in-tree version of JuliaSyntax (JuliaLowering
to be hooked up later).
Also remove JuliaLowering / JuliaSyntax GHA actions and update
JuliaLowering `[sources]` to use local `JuliaSyntax`.
