Restore crossbuild task desc and remove internal task desc

Author: kateinoigakukun

lib/ruby_wasm/build_system/product/baseruby.rb

@@ -26,7 +26,6 @@ def name
     def define_task
       directory product_build_dir
 
-      desc "build baseruby #{@channel}"
       @install_task =
         task name => [
                source.src_dir,

lib/ruby_wasm/rake_task.rb

@@ -73,6 +73,7 @@ def initialize(
 
     @crossruby.define_task
 
+    desc "Cross-build Ruby for #{@target}"
     task name do
       next if @crossruby.built?
       @crossruby.build