Assume spidermonkey path is set correctly in ~/.emscripten

Following #2952, we know that SPIDERMONKEY_ENGINE has already been set in ~/.emscripten once a file has done `import shared` (if it hadn't we'd already have an error by that point).

So I think it makes sense to rely on the .emscripten file being the single point of truth, rather than trying a bit harder to find spidermonkey just for this single bit of functionality.

This PR should be reviewed/merged **AFTER** #2952 is merged (otherwise the diffs will include that PR).