build-script: always use the just-built swift compiler to build SwiftSyntax.
build-script: always use the just-built swift compiler to build SwiftSyntax.

Since we can now use SwiftPM from the build environment to build SwiftSyntax,
we need to teach SwiftPM to always use the just-built swift compiler instead
of the one from the environment. This ensures the binary compatibility of the
SwiftSyntax dylib and the rest part of the stdlib.

apple/swift-syntax,apple/swift-syntax,apple/swift-syntax