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)