Installing dependencies before build fails when using uv and disabling build isolation

[burgholzer]

Description

According to the cibuildwheel documentation, it is valid to disable build isolation for the wheel builds via

build-frontend = { name = "build[uv]", args = ["--no-isolation"] }

Naturally, this requires to install the build time dependencies manually.
However, specifying

before-build = "uv pip install scikit-build-core[pyproject]>=0.8.1 setuptools_scm>=7 pybind11>=2.13"

and then running cibuildwheel results in

Building cp38-manylinux_x86_64 wheel
CPython 3.8 manylinux x86_64

Setting up build environment...

    + /opt/python/cp38-cp38/bin/python -c 'import sys, json, os; json.dump(os.environ.copy(), sys.stdout)'
    + which python
    + which uv

                                                              ✓ 0.07s
Running before_build...

    + sh -c 'uv pip install scikit-build-core[pyproject]>=0.8.1 setuptools_scm>=7 pybind11>=2.13'
error: No virtual environment found

                                                              ✕ 3.16s
Error: Command ['sh', '-c', 'uv pip install scikit-build-core[pyproject]>=0.8.1 setuptools_scm>=7 pybind11>=2.13'] failed with code 2. 

This only seems to be happening under linux because for all the other operating systems, cibuildwheel explicitly calls uv venv ... in the "Setting up build environment..." step.

Build log

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CI config

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