fix: use nix 2.29.1 for ephemeral installs

We will upgrade in the future, but for now we need to stick to 2.29.1

Author: jfroche

.github/actions/nix-install-ephemeral/action.yml

@@ -39,7 +39,7 @@ runs:
     - name: Install nix
       uses: cachix/install-nix-action@v31
       with:
-        install_url: https://releases.nixos.org/nix/nix-2.31.2/install
+        install_url: https://releases.nixos.org/nix/nix-2.29.1/install
         extra_nix_config: |
           substituters = https://cache.nixos.org https://nix-postgres-artifacts.s3.amazonaws.com
           trusted-public-keys = nix-postgres-artifacts:dGZlQOvKcNEjvT7QEAJbcV6b6uk7VF/hWMjhYleiaLI= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=