Stop creating `Modules/Setup.local` when doing WASI builds

[brettcannon]

Tools/wasm/wasi has been setting Modules/Setup.local because that's how it was done before. But it seems to be unnecessary, so we should stop doing it (while warning when the file does exist).

Linked PRs

• GH-137049: Stop creating an empty Modules/Setup.local when doing WASI builds #137050

[brettcannon]

Turns out that extension loading breaks w/o the empty Setup.local but which is no triggered w/ python.wam --version.