Remove the dedicated `zig build lib-vt` step and replace it with a `-Demit-lib-vt` build option. This fixes two problems: 1. We can default XCFramework, app, etc. steps to false if emit-lib-vt is true, so that the lib-vt build doesn't pull in unrelated artifacts. **Most importantly, lib-vt alone can be build without full Xcode installations.** 2. We can build lib-vt as part of a bundle with other artifacts if we really want.