See if we can use --scratch-path to speed up samples building

[ktoso]

Because we keep re-building swift syntax 6+ times right now, which is a waste.

We could swift build --scratch-path=../../.build and adjust the plugins to consider scratch paths when looking for dylibs hmmm

[ktoso]

Alternatively it seems we can tell swiftpm about a workspace file: https://github.com/swiftlang/swift/blob/990d62c099aa1436e43fb1765a2c3b948230a54d/utils/build_swift/resources/SwiftPM-Unified-Build.xcworkspace/contents.xcworkspacedata

like this: https://github.com/search?q=repo%3Aswiftlang%2Fswift%20multiroot&type=code

and then they're all considered in the same build and may avoid rebuilding