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>

Author: vincentkfu

ci/actions-install.sh

@@ -84,8 +84,9 @@ install_macos() {
     #echo "Updating homebrew..."
     #brew update >/dev/null 2>&1
     echo "Installing packages..."
-    HOMEBREW_NO_AUTO_UPDATE=1 brew install cunit libnfs
-    pip3 install scipy six sphinx
+    HOMEBREW_NO_AUTO_UPDATE=1 brew install cunit libnfs sphinx-doc
+    brew link sphinx-doc --force
+    pip3 install scipy six 
 }
 
 main() {