build-script: use libSwiftSyntax.dylib instead of libswiftSwiftSyntax.dylib.
build-script: use libSwiftSyntax.dylib instead of libswiftSwiftSyntax.dylib.

We used to install SwiftSyntax as if it's part of the stdlib so we had to use the latter
name.

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