chore: install brew and neovim in ci

.github/workflows/ci.yml

@@ -18,6 +18,12 @@ jobs:
       - uses: fish-actions/fisher@v1
         with:
           plugins: jorgebucaran/fishtape ilancosman/clownfish $GITHUB_WORKSPACE
+      - name: Set up Homebrew
+        uses: Homebrew/actions/setup-homebrew@master
+        # Homebrew is only missing from ubuntu images per https://github.com/actions/runner-images/issues/6283
+        if: matrix.os == 'ubuntu-latest'
+      - name: Install neovim
+        run: brew install neovim
       - name: Run full test suite
         run: fishtape tests/*.fish
         shell: fish {0}