Update check action to use ubuntu-latest (#902)

Author: raven42

.github/workflows/check.yml

@@ -7,7 +7,7 @@ jobs:
       matrix:
         vimFlavor: ["vim", "nvim"]
         tagsProvider: ["exuberant-ctags", "universal-ctags"]
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
     - name: Checkout
       uses: actions/checkout@v2