Fix nix version to 2.18.1, known working version

Author: RCoeurjoly

.github/workflows/nix-github-actions.yml

@@ -10,4 +10,6 @@ jobs:
       with:
        submodules: true
     - uses: cachix/install-nix-action@v26
-    - run: nix build '.?submodules=1'
+      with:
+       install_url: https://releases.nixos.org/nix/nix-2.18.1/install
+    - run: nix build .?submodules=1