fix: update root go.mod in prebuild.sh to resolve CI test failures (#3069)

tools/hack/prebuild.sh

@@ -18,9 +18,6 @@ for repo in ${envoy_repos[@]}; do
     cp -RP envoy/$repo  external/$repo
     cd external/$repo
     echo "gitdir: /parent/.git/modules/envoy/$repo" > .git
-    if [ -f "go.mod" ]; then
-        go mod tidy -go=${GO_VERSION}
-    fi
     cd $WORK_DIR
 done
 
@@ -33,8 +30,8 @@ for repo in ${istio_repos[@]}; do
     cp -RP istio/$repo external/$repo
     cd external/$repo
     echo "gitdir: /parent/.git/modules/istio/$repo" > .git
-    if [ -f "go.mod" ]; then
-        go mod tidy -go=${GO_VERSION}
-    fi
     cd $WORK_DIR
 done
+
+# Update root go.mod after all external dependencies are ready
+go mod tidy