Test markdown changelog creation in CI

Extract a github action to install the same version used in the release
workflow.  In future we should probably migrate to requirements.txt
or similar.

.github/actions/install-sphinx-markdown-builder/action.yml

@@ -0,0 +1,10 @@
+name: Install sphinx-markdown-builder
+
+permissions:
+  contents: read
+
+runs:
+  using: "composite"
+  steps:
+    - run: pip install sphinx-markdown-builder==0.6.8
+      shell: bash

.github/workflows/main.yml

@@ -22,6 +22,7 @@ jobs:
         sudo apt install gettext libpcre2-dev python3-pexpect python3-sphinx tmux
         # Generate a locale that uses a comma as decimal separator.
         sudo locale-gen fr_FR.UTF-8
+    - uses: ./.github/actions/install-sphinx-markdown-builder
     - name: cmake
       run: |
         mkdir build && cd build

.github/workflows/release.yml

@@ -42,12 +42,12 @@ jobs:
           ref: ${{ inputs.version }}
       - name: Install dependencies
         run: sudo apt install cmake gettext ninja-build python3-pip python3-sphinx
+      - uses: ./.github/actions/install-sphinx-markdown-builder
       - name: Create tarball
         run: |
           set -x
           mkdir /tmp/fish-built
           FISH_ARTEFACT_PATH=/tmp/fish-built ./build_tools/make_tarball.sh
-          pip install sphinx-markdown-builder==0.6.8
           relnotes=/tmp/fish-built/release-notes.md
           sh -x ./build_tools/release-notes.sh >"$relnotes"
           # Delete title