bug(build): pixi-build-python does not expand $PREFIX in Windows build environment

[juenglin]

Checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pixi, using pixi --version.

Reproducible example

Minimal reproduction for a Windows pixi-build bug: [package.build.config.env]
values that reference $PREFIX are written literally into rattler-build's
build_env.bat instead of being expanded to absolute paths.

Reproduce

pixi install -v

Expected failure today:

RuntimeError: CUDA_HOME was not expanded on Windows: '$PREFIX/Library'

Inspect the generated build environment:

type .pixi\bld\prefix-env-repro\*\work\build_env.bat | findstr /i "PREFIX CUDA_HOME"

Expected broken output:

@SET "PREFIX=C:\...\host"
@SET "LIBRARY_PREFIX=C:\...\host\Library"
@SET "CUDA_HOME=$PREFIX/Library"

pixi.toml:

[workspace]
channels = ["conda-forge"]
platforms = ["win-64"]
preview = ["pixi-build"]

[package]
name = "prefix-env-repro"
version = "0.1.0"

[package.build]
backend = { name = "pixi-build-python", version = "*" }

[package.host-dependencies]
python = "3.12.*"
setuptools = ">=80"

# Mirrors cuda-python's pattern for conda CUDA headers on Windows.
[package.build.target.win-64.config.env]
CUDA_HOME = "$PREFIX/Library"

pyproject.toml:

[build-system]
requires = ["setuptools>=80"]
build-backend = "build_hooks"
backend-path = ["."]

[project]
name = "prefix-env-repro"
version = "0.1.0"

build_hooks.py:

"""Minimal PEP 517 backend that asserts CUDA_HOME is expanded on Windows."""

import os

from setuptools import build_meta as _build_meta

prepare_metadata_for_build_wheel = _build_meta.prepare_metadata_for_build_wheel
get_requires_for_build_wheel = _build_meta.get_requires_for_build_wheel


def build_wheel(wheel_directory, config_settings=None, metadata_directory=None):
    cuda_home = os.environ.get("CUDA_HOME") or os.environ.get("CUDA_PATH")
    if not cuda_home:
        raise RuntimeError("CUDA_HOME/CUDA_PATH not set in build env")
    if "PREFIX" in cuda_home:
        raise RuntimeError(
            f"CUDA_HOME was not expanded on Windows: {cuda_home!r}. "
            f"PREFIX={os.environ.get('PREFIX')!r} "
            f"LIBRARY_PREFIX={os.environ.get('LIBRARY_PREFIX')!r}"
        )
    return _build_meta.build_wheel(wheel_directory, config_settings, metadata_directory)

Issue description

build.script.env placeholders like $PREFIX are not expanded when generating Windows build_env.bat

Expected behavior

build.script.env placeholders are expanded on Windows.

[hiSandog]

This looks like Unix-style environment expansion leaking into the Windows build path. A good regression should run the pixi-build-python environment resolution on Windows semantics and assert $PREFIX or the equivalent placeholder is expanded before invoking the backend. It would be worth testing both cmd and PowerShell-style environments if users can hit either through build scripts.