Faster tuist cache warm via parallel XCFramework creation

The “Creating XCFrameworks” phase of tuist cache warm now runs xcodebuild -create-xcframework across targets concurrently instead of one at a time. The fan-out is bounded by the host’s active processor count with a cap of 8, so projects with many cacheable framework targets complete the cache warm significantly faster while producing the same set of artifacts and failing the run if any single create-xcframework call fails.