fix nix flake check on macOS

.github/workflows/test-flakes.yml

@@ -16,7 +16,6 @@ jobs:
           fetch-depth: 0
     - uses: cachix/install-nix-action@v14
       with:
-        install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20201221_9fab14a/install
         extra_nix_config: |
           experimental-features = nix-command flakes
           system-features = nixos-test benchmark big-parallel kvm
@@ -27,5 +26,5 @@ jobs:
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
     - name: List flake structure
       run: nix flake show
-    - name: Run unit tests (flake)
-      run: nix build --no-link .#unit-tests -L
+    - name: Run flake check (flake)
+      run: nix flake check -L