ci: correctly checkout vim-flavor using https://

Currently, the CI throws:

The unauthenticated git protocol on port 9418 is no longer supported.
Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.

So update vim-flavor to 4.0.1 which uses https:// links instead of
un-authenticated git protocol. Unfortunately, we have to update ruby to
version 3.0 with that.

Let's see if this works.

.github/workflows/ci.yml

@@ -20,19 +20,23 @@ jobs:
         - v8.1.0000
         - v8.0.0000
         - v7.4
+
     steps:
     - name: Checkout code
       uses: actions/checkout@v2
+
     - name: Checkout vim-themis
       uses: actions/checkout@v2
       with:
         repository: thinca/vim-themis
         path: vim-themis
+
     - name: Setup Vim
       uses: rhysd/action-setup-vim@v1
       id: vim
       with:
         version: ${{ matrix.vim }}
+
     - name: Test
       env:
         THEMIS_VIM: ${{ steps.vim.outputs.executable }}