Experimental fix for mirage/mirage#221

This leaves a stray binary after an uninstall, but fixes the
situation where the `mirage` binary attempts to upgrade itself
(due to invoking OPAM, which installs some optional dependencies,
which then proceed to try and recompile mirage).

The only long-term fix is to lift `mirage` into being a host-level
tool and not tracked by the same tool it uses to install libraries,
but for the moment renaming the active `mirage` binary to `mirage.old`
on uninstall should do the trick.