rust: Skip target recipe since it doesn't work

rust: Skip target recipe since it doesn't work

The target rust recipe is known not to work. Add a SkipRecipe entry for
that so world builds don't include something known to be broken and
hence give users a sensible message if they do try and build it rather
than a build failure.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>