Environment is not correctly updated on Windows on toolchain installation

[winstxnhdw]

Bug Description

_get_env() should update os.environ instead of returning a merged dict

On Windows, passing a custom env dict to subprocess does not affect how the OS searches for the executable. This means subprocess.check_output(["cargo", "--version"], env={**os.environ, **extra_env}) fails to find cargo.exe even though the merged dict contains the correct PATH.

To fix this, I am guessing we should update os.environ in place instead of returning a merged dict:

def _get_env() -> Optional[Dict[str, str]]:
    if not os.environ.get("MATURIN_NO_INSTALL_RUST") and not shutil.which("cargo"):
        from puccinialin import setup_rust

        print("Rust not found, installing into a temporary directory")
        extra_env = setup_rust()
        os.environ.update(extra_env)
    return None

Related to konstin/puccinialin#16

Your maturin version (maturin --version)

1.12.6

Your Python version (python -V)

All Python versions

Your pip version (pip -V)

26.0.1

What bindings you're using

None

Does cargo build work?

• Yes, it works

If on windows, have you checked that you aren't accidentally using unix path (those with the forward slash /)?

• Yes

Steps to Reproduce

name: Install

on: [push]

jobs:
  install:
    runs-on: windows-latest

    steps:
      - name: Uninstall Rust
        run: rustup self uninstall -y

      - name: Install GxHash
        run: pip install gxhash

Workflow will fail when it should pass.

[winstxnhdw]

Hi @messense, could we have a new release for this since it literally prevents all Windows users from installing the toolchain?