Publish pre-built CPython tarball for WASI

Currently, building `componentize-py` requires building CPython from source for WASI (and for the native platform as well, due to the bootstrapping requirement). This is annoying because we rarely change CPython and eventually want to use official upstream builds anyway.

Once there are compatible upstream builds (i.e. `libpython3.14.so` for WASI), we can switch to them, but in the meantime we should consider publishing our own builds and using them instead by default of building CPython from source as part of the `componentize-py` build.

Note that we should still support building CPython by default as a build-time option (e.g. via an environment variable) for debugging purposes.