Improve rust devx
Some thoughts here on how to improve the experience for someone trying:

```
pip install py2many
py2many --rust=1 hello.py
```

I expect most rust devs to be using a recent stable toolchain:

```
$ rustup toolchain list
stable-x86_64-unknown-linux-gnu (default)
nightly-2024-01-01-x86_64-unknown-linux-gnu
```

However,

```
pytest -k hello_world-rust  # fails with

Compiling ['rustup', 'run', 'nightly-2024-01-01', 'cargo', 'build', '-Zscript', '--manifest-path', '/home/arun/src/py2many/tests/build/hello_world.rs'] ...
Invoking ['rustup', 'run', 'nightly-2024-01-01', 'cargo', 'run', '-Zscript', '--manifest-path', '/home/arun/src/py2many/tests/build/hello_world.rs'] ...
Running linter ['cargo', 'clippy', '-Zscript', '--manifest-path', '/home/arun/src/py2many/tests/build/hello_world.rs'] ...
---------------------------------------------------------------------------------------------------------------------------------- Captured stderr call -----------------------------------------------------------------------------------------------------------------------------------
Writing to: /home/arun/src/py2many/tests/build
   Compiling hello_world v0.0.0 (/home/arun/src/py2many/tests/build)
    Finished dev [unoptimized + debuginfo] target(s) in 2.20s
error: the `-Z` flag is only accepted on the nightly channel of Cargo, but this is the `stable` channel
```