Stop using a nix prerelease in CI.

Additionally, stop installing an extra copy of nix.

.github/workflows/CI_NixPy.yml

@@ -27,7 +27,7 @@ jobs:
       uses: cachix/install-nix-action@v13
       with:
         nix_path: nixpkgs=channel:nixos-unstable
-        install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20201221_9fab14a/install
+        install_url: https://releases.nixos.org/nix/nix-2.6.0/install
         extra_nix_config: |
           experimental-features = nix-command flakes
           sandbox = true
@@ -47,8 +47,6 @@ jobs:
 
     - name: mach-nix evaluation tests
       run: |
-        wget https://github.com/DavHau/nix-portable/releases/download/v008/nix-portable
+        WORKERS=5 nix run .#tests-all
-        chmod +x nix-portable
-        WORKERS=5 ./nix-portable nix run .#tests-all
       shell: bash