ci: remove help-check for Nvim nightly

Author: alex-courtis

.github/workflows/ci.yml

@@ -87,10 +87,12 @@ jobs:
       - run: make format-check
 
       - name: build Nvim from source
+        if: ${{ matrix.nvim_version == 'stable' }}
         run: |
           mkdir -p "${DIR_NVIM_SRC}"
           curl -L "https://github.com/neovim/neovim/archive/refs/tags/${{ matrix.nvim_version }}.tar.gz" | tar zx --directory "${DIR_NVIM_SRC}/.."
           cd "${DIR_NVIM_SRC}"
           make doc
 
       - run: make help-check
+        if: ${{ matrix.nvim_version == 'stable' }}