Add CacheDir support for CACHEDIR.TAG

[mwichmann]

A number of tools for backup understand a marker file named CACHEDIR.TAG as a signal that the contents of the directory it's found in are cache contents, which might hint at different handling. This file is described in a specification:

https://bford.info/cachedir/

Suggesting the SCons derived-file caching code could create this file as a low-cost improvement for tools that care about it. It's a static file with one required line and optional comments.