ci: checkout via Nix rather than action (#280)

This is closer to how users would install Stylix. It may also be
slightly faster since there's one less action to process.

.github/workflows/docs.yml

@@ -25,11 +25,8 @@ jobs:
       - name: Set up cache
         uses: DeterminateSystems/magic-nix-cache-action@main
 
-      - name: Checkout repository
-        uses: actions/checkout@v4
-
       - name: Build docs
-        run: nix -L build .#docs
+        run: nix -L build github:${{ github.repository }}/${{ github.sha }}#docs
 
       - name: Prepare docs for upload
         run: cp -r --dereference --no-preserve=mode,ownership result/ public/