cargo: remove True option to getVar calls

Layer cleanup similar to
https://git.openembedded.org/openembedded-core/commit/?id=26c74fd10614582e177437608908eb43688ab510



Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 9a2ed524)
Signed-off-by: Steve Sakoman <steve@sakoman.com>