triagebot: add jieyouxu to fallback adhoc_group

So that there's `n > 1` selection pool in case one of the eligible
reviewers go off rotation.

Author: jieyouxu

triagebot.toml

@@ -1532,7 +1532,8 @@ debuginfo = [
     "@davidtwco"
 ]
 fallback = [
-    "@Mark-Simulacrum"
+    "@Mark-Simulacrum",
+    "@jieyouxu",
 ]
 style-team = [
     "@calebcartwright",