Move off nightly for Python guest

# What
#422 (re-)introduced the need to use a nightly compiler for our Python guest. Make this use the stable compiler again.

# Why
Similar to #19:

- Not having to worry about a 2nd compiler version is nice.
- Some of our unstable flags might break during upgrades

# How
The following things have to work on stable first:

- [ ] [`build-std`](https://doc.rust-lang.org/cargo/reference/unstable.html#build-std): required because the std lib shipped with rust isn't built with `-Crelocation-model=pic`, which we need for linking shared libraries
- [ ] [`default-visibility`](https://doc.rust-lang.org/stable/unstable-book/compiler-flags/default-visibility.html#default-visibility): required to drastically shrink the size of the resulting WASM binary, also see https://github.com/bytecodealliance/componentize-py/pull/209

Start by reviewing #422 and the related discussion in #19 to understand why the Python guest currently needs nightly Rust. Check stable support for build-std and default-visibility, including the linked componentize-py pull request #209. Done means the Python guest builds successfully with stable Rust while retaining the required relocation model and acceptably small WASM output.

Written by the indexing model from the issue text.