sdk/project.conf: Fix ca_path to not be outside /usr

We should avoid using /etc and use /usr/etc instead. If you use this variable to install the file at this path then we get a troubles. If there is a change of order on how dependencies are composed, then we get /etc to be created as a directory. Then another dependency tries to create it as a symbolic link, and buildstream stops with some cryptic error. We already had that kind of issue some months ago.