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/actions/install-sphinx/action.yml

@@ -9,12 +9,14 @@ runs:
     - shell: bash
       run: |
         set -x
-        sudo pip install uv --break-system-packages
+        pip=$(command -v pip pip3 | head -1)
+        sudo "$pip" install uv --break-system-packages
         # Check that pyproject.toml and the lock file are in sync.
         # TODO Use "uv" to install Python as well.
         : 'Note that --no-managed-python below would be implied but be explicit'
-        uv --no-managed-python lock --check
+        uv='env UV_PYTHON=python uv --no-managed-python'
+        $uv lock --check
         # Install globally.
-        sudo uv --no-managed-python pip install --group=dev --system --break-system-packages
+        sudo $uv pip install --group=dev --system --break-system-packages
         # Smoke test.
         python -c 'import sphinx; import sphinx_markdown_builder'

.github/workflows/release.yml

@@ -152,6 +152,7 @@ jobs:
           targets: aarch64-apple-darwin,x86_64-apple-darwin
       - name: Install dependencies
         run: brew install gettext
+      - uses: ./.github/actions/install-sphinx
       - name: Build and codesign
         run: |
           die() { echo >&2 "$*"; exit 1; }

.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