Only install SwiftSyntax when we can build it

Author: DougGregor

utils/swift_build_support/swift_build_support/products/earlyswiftsyntax.py

@@ -83,7 +83,7 @@ def should_install(self, host_target):
         Whether or not this product should be installed with the given
         arguments.
         """
-        return self.args.install_swiftsyntax
+        return self.should_build(host_target) and self.args.install_swiftsyntax
 
     def install(self, host_target):
         """