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.
3fc04fd4ae