Linking merge requests to failed deployments prevents them from being
linked to future deployments, as we only link a merge request once per
environment. In addition, linking to failed deployments isn't very
useful, as an MR technically isn't deployed (because the deploy failed).

This commit changes our MR linking logic so we don't link merge requests
for deployments that did not succeed.

See https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1659 for
more information.