Install Sphinx in macOS CI/CD

Looks like macOS packages didn't have docs??
I noticed via our Cargo warning about sphinx-build.

macOS has a variety of pythons installed:

	bash-3.2$ type -a python python3
	python is /Library/Frameworks/Python.framework/Versions/Current/bin/python
	python3 is /opt/homebrew/bin/python3
	python3 is /usr/local/bin/python3
	python3 is /Library/Frameworks/Python.framework/Versions/Current/bin/python3
	python3 is /usr/bin/python3

by default, "uv --no-managed-python" uses python3 (homebrew), but
"python" (and "pip") use the other one.  Make uv use the same as
our scripts.  Not all uv commands support "--python" today, so set
UV_PYTHON.

.github/workflows/test.yml

@@ -136,6 +136,7 @@ jobs:
         # this is CI so we don't actually care.
         sudo pip3 install --break-system-packages pexpect
         brew install gettext tmux
+    - uses: ./.github/actions/install-sphinx
     - name: cmake
       run: |
         mkdir build && cd build
@@ -147,6 +148,9 @@ jobs:
     - name: make fish_run_tests
       run: |
         make -C build VERBOSE=1 fish_run_tests
+    - name: Setup tmate session
+      if: ${{ failure() }}
+      uses: mxschmitt/action-tmate@v3
 
   windows:
     runs-on: windows-latest