Add environment health as another root cause for release blockers

Per discussion gitlab-org/release/tasks#21137 (comment 2728409931), we found that ~"RootCause::Flaky-Test" label can be misleading sometimes when the reason of failure is not test behavior related. This issue is to request that we don't assign ~"RootCause::Flaky-Test" label to release blocker issue if no tests were run due to environment health's related issue.