Enable read/write access to dune's cache directory when sandboxing.

[mefyl]

Currently Opam's sandboxing prevent dune caching mechanism from working, as sandboxing removes write access to most of the filesystem.

The currently proposed solution would be to simply add the cache directory to the list of rw directiories in sandbox.sh. The value on Unix and OSX would be $XDG_CACHE_HOME/dune if set, $HOME/.cache/dune otherwise. Windows would be $AppData/Local Settings/Cache, although I'm not sure sandboxing works there.

[rjbou]

Thanks for reporting!