ci: Disable nightly tests until Neovim nightly is fixed

Once neovim/neovim#30792 is fixed, nightly
tests can be re-enabled.

Author: kristijanhusak

.github/workflows/tests.yml

@@ -29,7 +29,6 @@ jobs:
           - v0.10.0
           - v0.10.1
           - v0.10.2
-          - nightly
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4