This takes about twice as long to build in CI, but runs significantly
quicker (~22x). The exact time taken varies quite a bit between runs,
but this brings the worst-case overall run-time down from ~500s to
~200s. While here, make sure we pass `--skip-build` to the second
invocation to ensure we avoid building twice. Also disable using the
dependency cache since we want to test a clean build, and we just
want swiftpm to use the local deps.
This is a tool specifically designed to generate
Xcode projects for the Swift repo (as well as a
couple of adjacent repos such as LLVM and Clang).
It aims to provide a much more user-friendly experience
than the CMake Xcode generation (`build-script --xcode`).
