debian: Only build with Werror when we are doing a native build

-Werror is generally considered unsuitable for Debian since it can cause build failures when dependencies are updated, like in #13

On the other hand, it might be a useful option for upstream development.

The ubports packaging is 3.0 (native) while Debian's packaging is 3.0 (quilt) so we can use that detail to set the build option (and keep debian/rules synced with Debian).

Also use some similar logic to set the hybris build flag.

On the other hand, if we don't run CI automatically periodically, using -Werror even for ubports packaging might result in unexpected CI failures when someone tries to update the project's code.