for_eachで対応する。
下手なnested loopで対応しようとすると追加時に毎回差分が出るので、これで対応

locals {  
  developer_roles = [  
    "roles/container.viewer",  
    "roles/logging.viewer",  
  ]  
  developers = [  
    "hogehoge@example.com"  
  ]  
  developers_with_developer_roles = { for temp in flatten([  
    for developer in local.developers : [  
      for developer_role in local.developer_roles : {  
        developer      = developer  
        developer_role = developer_role  
      }  
    ]  
  ]) : "${temp.developer}-${temp.developer_role}" => temp }  
}  

resource "google_project_iam_member" "developer" {  
  for_each = local.developers_with_developer_roles  
  role     = each.value.developer_role  
  member   = "user:${each.value.developer}"  
}