Emscripten: Fix try_run to run the `.js` file and not the adjacent `.wasm`

Since commit 96d9b94a98 (Emscripten: Add platform modules, 2025-05-16,
v4.2.0-rc1~607^2~3) we've considered the `.wasm` to be the `try_compile`
output because we need `COPY_FILE` to get it for extracting `INFO:`
strings during our inspection checks.  This breaks `try_run` because
`node`, used via `CMAKE_CROSSCOMPILING_EMULATOR`, expects the `.js`.
Revert to considering the `.js` to be the primary output file, but
switch to the `.wasm` in `COPY_FILE`'s implementation.

Fixes: #27421