Cosign.gitlab-ci.yml template doesn't fail the job when cosign produces an error

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From slack thread: https://gitlab.slack.com/archives/C04SB6GR2D8/p1696406118754609?thread_ts=1696346641.087589&cid=C04SB6GR2D8

Btw when I changed to the template, the job now succeeds but the signing fails.I imagine we'd want the job to fail? https://gitlab.com/gl-demo-ultimate-paporte/leticket-app/-/jobs/5219063074

This is because the template runs cosign in after_script, which doesn't affect the job's exit code.

We should fail the job in this case.