Check modified time when reusing tmp dir

I recommend that an executable file check its modified time and that of `tmp` dir, so that old `tmp` dir is not be used if the executable file is newer than `tmp` dir.

Consider the case below,

* compile with `mix release --executable`
* execute `_build/prod/rel//bin/.run`
* `tmp` dir is created
* recompile
* execute newly created `_build/prod/rel//bin/.run`
* exe files in the old `tmp` dir are used

In this case, any modifications are not reflected to the executable files.
I think it is an unexpected behavior, especially for new users.