Mostly I was copying the approach we use for the shopify_function Rust crate here.

We could commit the trampolined file. The part that kind of sucks with rebuilding the Wasm file manually is getting the correct version of the trampoline since if you grab the wrong version, the trampolining will appear to work but running it will fail with a failed to instantiate error. I could also update the test such that we check if the trampoline file exists and don't bother downloading and running the trampoline if it does and then update the instructions for rebuilding the fixtures to say delete the trampolined version and run the tests to rebuild it. Or I could write a Makefile to handle building the fixtures and that could handle downloading and running the correct trampoline version. Thoughts?