Allow specifying cachedir from cmdline

[mwichmann]

After a brief mention on Discord (sadly can't link to those)

currently can only enable derived-file caching with a call to CacheDir(path) in an sconscript. It seems like a command-line option to enable caching globally, and set the file - in other words, equivalent to calling the global CacheDir (but not to calling env.CacheDir), would be useful. This would make the feature more equivalent to Repository / --repository=path

[mwichmann]

While on the subject of the cachedir, should there be an option to prune the cache (flush old entries)?

[acmorrow]

@mwichmann - Yes, pruning functionality for the cache is definitely needed. Otherwise you inevitably fill your storage with dead cached objects.