ci(external): no need for neovim-ppa/stable #39557

Problem: The `with-external-deps` workflow keeps failing because
adding the neovim-ppa/stable times out.

Solution: Don't add the PPA; it doesn't look to be necessary for
installing current dependencies.

.github/workflows/test.yml

@@ -320,7 +320,6 @@ jobs:
 
       - name: Install dependencies
         run: |
-          sudo add-apt-repository ppa:neovim-ppa/stable
           sudo apt-get install -y \
             libluajit-5.1-dev \
             libunibilium-dev \