fix(ci): macOS CI fails with "Unrecognized archive format"

    Run curl -LO 'https://github.com/neovim/neovim/releases/download/nightly/nvim-macos.tar.gz'
    tar: Error opening archive: Unrecognized archive format

ref: neovim/neovim@036f86f

Author: justinmk

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
           NVIM_BIN_PATH: nvim-linux64/bin
           EXTRACT: tar xzf
         - os: macos-11
-          NIGHTLY: nvim-macos.tar.gz
+          NIGHTLY: nvim-macos-x86_64.tar.gz
           NVIM_BIN_PATH: nvim-macos/bin
           EXTRACT: tar xzf
         - os: windows-latest