invoke xcodebuild from the proper xcode_dir

Author: lrz

lib/motion/project/template/ios-extension-config.rb

@@ -200,7 +200,7 @@ def supported_sdk_versions(versions)
     def sdk_build_version(platform)
       @sdk_build_version ||= begin
         sdk_path = sdk(platform)
-        `xcodebuild -version -sdk '#{sdk_path}' ProductBuildVersion`.strip
+        `\"#{xcode_dir}/usr/bin/xcodebuild\" -version -sdk '#{sdk_path}' ProductBuildVersion`.strip
       end
     end