Check modified time when reusing tmp dir

[s417-lama]

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/<app>/bin/<app>.run
• tmp dir is created
• recompile
• execute newly created _build/prod/rel/<app>/bin/<app>.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.

[bitwalker]

Seems like a good change to make, I'm happy to merge a PR for that, but I won't have time to get to it and test it myself for awhile unfortunately.