github: use rhysd/action-setup-vim action

Author: zchee

.github/workflows/test.yml

@@ -26,16 +26,10 @@ jobs:
       uses: actions/checkout@v2
 
     - name: Install nvim binary
-      run: |
-        case "$RUNNER_OS" in
-          Linux)
-            curl -sSL https://github.com/neovim/neovim/releases/download/nightly/nvim-linux64.tar.gz | sudo tar xfz - --strip-components=1 -C '/usr/local'
-          ;;
-          macOS)
-            curl -sSL https://github.com/neovim/neovim/releases/download/nightly/nvim-macos.tar.gz | sudo tar xfz - --strip-components=1 -C '/usr/local'
-          ;;
-        esac
-        nvim --version
+      uses: rhysd/action-setup-vim@v1
+      with:
+        neovim: true
+        version: nightly
 
     - name: Test
       run: |