ci: use Neovim nightly instead of master

...to allow more caching

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        neovim_branch: ['v0.6.1', 'v0.7.0', 'master']
+        neovim_branch: ['v0.6.1', 'v0.7.0', 'nightly']
     # The type of runner that the job will run on
     runs-on: ubuntu-latest
     env:
@@ -73,12 +73,17 @@ jobs:
       - name: Check lua files are built from latest teal
         run: make tl-ensure
 
+      - name: Get Neovim SHA
+        id: get-nvim-sha
+        run: |
+          echo "::set-output name=sha::$(git ls-remote https://github.com/neovim/neovim ${{env.NEOVIM_BRANCH}} | cut -f1)"
+        shell: bash
+
       - name: Cache neovim
         uses: actions/cache@v2
-        if: env.NEOVIM_BRANCH != 'master'
         with:
           path: deps/neovim
-          key: build-${{env.NEOVIM_BRANCH}}
+          key: build-${{ steps.get-nvim-sha.outputs.sha }}
 
       - name: Build Neovim
         run: make deps/neovim NEOVIM_BRANCH=$NEOVIM_BRANCH