test: use homebrew to install sphinx instead of pip on macOS

test: use homebrew to install sphinx instead of pip on macOS

With the current GitHub Actions macOS image, pip3 install sphinx does
not appear to place sphinx-doc in the path. This results in
documentation build failures.

Resolve this by using homebrew to install sphinx-doc and add it to the
search path.

https://www.sphinx-doc.org/en/master/usage/installation.html
https://github.com/vincentkfu/fio/actions/runs/3395703049/jobs/5645918799

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>