fileutils: use rm_rf to clean temp dir

Honestly, I don't know why git is broken under sandbox. But this seems
fix the problem in Homebrew/homebrew#38978.

Closes Homebrew/homebrew#39138.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/extend/fileutils.rb

@@ -19,7 +19,7 @@ module FileUtils
         cd(prev)
       end
     ensure
-      ignore_interrupts { rm_r(tmp) }
+      ignore_interrupts { rm_rf(tmp) }
     end
   end
   module_function :mktemp