gitlab-ci: Use Werror for the cross build

We allow the build to fail since we're mostly interested in
the debs but this should alert us when builds throw errors.

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
2 jobs for imx8-current-librem5
in 62 minutes and 40 seconds, using 0.0 compute credits, and was queued for 2 seconds