vlock: Don't try to install setuid binary

With newer Nix it's (fortunately) no longer possible to create a file
with setuid bits, even though the permissions are fixed later the build
will fail during installPhase already.

I've verified whether the contents of the output path are the same as
before this change and the contents match.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>