This passes command line arguments to codegen, allowing in particular
`--force` to be passed.
Also, a convenience `//rust/codegen:py` is added to only run the python
based code generation, which will be faster and enough when
`ast-generator` is unchanged.
We've hardcoded the tilde in a lot of places :(
This improves performance on Windows and gets us ready for Bazel 8.
We need an upgrade of rules_rust for this to work.
