Local doc build claims sphinx not installed, when it is

[dstansby]

Minimal, reproducible code sample, a copy-pastable example if possible

cd docs
make html

Problem description

Running the above code I get:

Makefile:12: *** The 'sphinx-build' command was not found. Make sure you have Sphinx installed, then set the SPHINXBUILD environment variable to point to the full path of the 'sphinx-build' executable. Alternatively you can add the directory with the executable to your PATH. If you don\'t have Sphinx installed, grab it from http://sphinx-doc.org/.  Stop.

But I have sphinx installed!

> which sphinx-build
/Users/dstansby/mambaforge/envs/numcodecs/bin/sphinx-build

Version and installation information

Please provide the following:

• Value of numcodecs.__version__: current main branch
• Version of Python interpreter: 3.11
• Operating system (Linux/Windows/Mac): Mac
• How NumCodecs was installed (e.g., "using pip into virtual environment", or "using conda"):

Also, if you think it might be relevant, please provide the output from pip list or
conda list depending on which was used to install NumCodecs.

[QuLogic]

The check added in #451 is not portable; see e.g., https://unix.stackexchange.com/questions/85249/why-not-use-which-what-to-use-then