Skip build after build:download_prebuilt

Author: kateinoigakukun

lib/ruby_wasm/rake_task.rb

@@ -68,6 +68,7 @@ def initialize(
 
     desc "Cross-build Ruby for #{@target}"
     task name do
+      next if File.exist? @crossruby.artifact
       @crossruby.build
     end
     namespace name do