github/action: actually run the action and fork it for security

.github/main.workflow

 action "Auto Assign" {
-  uses = "kentaro-m/auto-assign@v1.0.0"
+  uses = "ipfs/auto-assign@v1.0.0"
+  secrets = ["GITHUB_TOKEN"]
+}
+workflow "Add reviewers/assignees to Pull Requests" {
+  on = "pull_request"
+  resolves = "Auto Assign"
 }