Rely on mtime to recompile app

[josevalim]

No description provided.

[dvic]

Won't this introduce the CI build cache issues as before? (#12056 (comment))

[josevalim]

It should not make a difference. compile.elixir and compile.erlang already relies on mtime as well. That issue was more about avoid recompiling when the mtime changed but the contents were the same, but we still relied on mtime to see if something maybe changed.