wasm: fix safari console error message related to import("node:timers")

This patch fixes the wasm backend JSFFI prelude script to avoid
calling `import("node:timers")` on non-deno hosts. Safari doesn't like
it and would print an error message to the console. Fixes
https://gitlab.haskell.org/ghc/ghc-wasm-meta/-/issues/13.

(cherry picked from commit 301c3b54)

utils/jsffi/prelude.mjs

@@ -58,9 +58,9 @@ const setImmediate = await (async () => {
   }
 
   // deno
-  try {
+  if (globalThis.Deno) {
     return (await import("node:timers")).setImmediate;
-  } catch {}
+  }
 
   // https://developer.mozilla.org/en-US/docs/Web/API/Scheduler/postTask
   if (globalThis.scheduler) {