Disable CI testing on Nightly.

Details:
- At the moment 'mini.test' on it fails to correctly compute child
  process screenshot for Command line area. See neovim/neovim#21886.

.github/workflows/tests.yml

@@ -15,7 +15,8 @@ jobs:
     timeout-minutes: 10
     strategy:
       matrix:
-        neovim_version: ['v0.6.1', 'v0.7.2', 'v0.8.2', 'nightly']
+        # neovim_version: ['v0.6.1', 'v0.7.2', 'v0.8.2', 'nightly']
+        neovim_version: ['v0.6.1', 'v0.7.2', 'v0.8.2']
 
     steps:
       - uses: actions/checkout@v3