[browser][mono] ST build is always sidecar on a worker (#121879)

Author: pavelsavara

src/mono/browser/runtime/loader/globals.ts

@@ -4,6 +4,8 @@
 /* eslint-disable @typescript-eslint/triple-slash-reference */
 /// <reference path="../types/sidecar.d.ts" />
 
+import WasmEnableThreads from "consts:wasmEnableThreads";
+
 import { exceptions, simd, relaxedSimd } from "wasm-feature-detect";
 
 import gitHash from "consts:gitHash";
@@ -17,8 +19,8 @@ import { mono_log_error, set_thread_prefix, setup_proxy_console } from "./loggin
 import { invokeLibraryInitializers } from "./libraryInitializers";
 import { deep_merge_config, isDebuggingSupported } from "./config";
 
-// if we are the first script loaded in the web worker, we are expected to become the sidecar
-if (typeof importScripts === "function" && !globalThis.onmessage) {
+// if we are ST build or the first script loaded in the web worker, we are expected to become the sidecar
+if (typeof importScripts === "function" && (!WasmEnableThreads || !globalThis.onmessage)) {
     (globalThis as any).dotnetSidecar = true;
 }