Update GitHub Actions to use Neovim 0.9.1.

Author: echasnovski

.github/workflows/tests.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        neovim_version: ['v0.7.2', 'v0.8.3', 'v0.9.0', 'nightly']
+        neovim_version: ['v0.7.2', 'v0.8.3', 'v0.9.1', 'nightly']
 
     steps:
       - uses: actions/checkout@v3