Prefer RbConfig.ruby over the 3.times fallback (#8691)

It seems saner to use RbConfig.ruby than using ruby in a random ancestor
directory.

tool/lib/envutil.rb

@@ -15,8 +15,10 @@ end
 module EnvUtil
   def rubybin
     if ruby = ENV["RUBY"]
-      return ruby
-    end
+      ruby
+    elsif defined?(RbConfig.ruby)
+      RbConfig.ruby
+    else
       ruby = "ruby"
       exeext = RbConfig::CONFIG["EXEEXT"]
       rubyexe = (ruby + exeext if exeext and !exeext.empty?)
@@ -29,9 +31,6 @@ module EnvUtil
         end
         ruby = File.join("..", ruby)
       end
-    if defined?(RbConfig.ruby)
-      RbConfig.ruby
-    else
       "ruby"
     end
   end