.gitlab-ci.yml: Allow DCO check to fail

Due to a known issue[1] our DCO checker will fail for commits that have a mismatching email for SoB and Author. For now, make this job optional.

[1] oniro#499

.gitlab-ci.yml

@@ -19,6 +19,8 @@ include:
 
 dco:
   extends: .dco
+  # https://gitlab.eclipse.org/eclipse/oniro-core/oniro/-/issues/499
+  allow_failure: true
 
 reuse:
   extends: .reuse