_platform/darwin.py: Give reason for use of dummy sandbox

Since Darwin is limited to the dummy sandbox for now due to OSXFUSE
being unsupported as yet, and there being no suitable sandboxing option
for use on OSX, give this as the reason so that it can be reported if
the user attempts a local build.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>