N+1 SQL queries in MergeRequestsController#ci_environment_status

On GitLab.com, there are significant number of slow endpoints coming from MergeRequestsController#ci_environment_status: https://log.gitlab.net/goto/52d53c5de5dd9d3300e434234c620553

The performance bar shows close to 1000 SQL queries retrieving clusters:

SQL backtraces:

Followup:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/63952
• https://gitlab.com/gitlab-org/gitlab-ce/issues/64105