unpack: preserve file attributes like mtime

Closes Homebrew/homebrew#48068.

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

Library/Homebrew/cmd/unpack.rb

@@ -28,7 +28,7 @@ module Homebrew
       ENV["VERBOSE"] = "1" # show messages about tar
       f.brew do
         f.patch if ARGV.flag?("--patch")
-        cp_r getwd, stage_dir
+        cp_r getwd, stage_dir, :preserve => true
       end
       ENV["VERBOSE"] = nil