[BUG]: pybind11-config sometimes give wrong result

What version (or hash if on master) of pybind11 are you using?

latest

Problem description

when I install matplotlib in editable mode, with pybind11 installed in --user home directory but not globally, error about header not found is raised.

context: https://github.com/matplotlib/matplotlib/issues/29741#issuecomment-2830629159

Update: as mentioned in the linked issue, this is probably an issue with pip https://github.com/pypa/pip/issues/13222

to reproduce:

# make sure pybind11 is not installed globally
pip install --user pybind11
git clone --depth 1 https://github.com/matplotlib/matplotlib
cd matplotlib
pip install -r requirements/dev/build-requirements.txt
pip install --verbose --no-build-isolation --editable ".[dev]"

Is this a regression? Put the last known working version here if it is.

Not a regression

Start by reproducing the editable matplotlib installation with the commands in the issue, checking the result of pybind11-config when pybind11 is installed with --user. Read the linked matplotlib context and pip issue #13222 to determine whether the failure belongs to pybind11 or pip; done means identifying a confirmed pybind11-side change or documenting that no change is needed here.