fix: assure nix version we need is installed at this stage + perm fix

Author: samrose

.github/workflows/testinfra-ami-build.yml

@@ -25,9 +25,6 @@ jobs:
             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=
 
-      - name: Fix nix permissions
-        run: sudo chown -R $USER /nix/var/nix/profiles/per-user
-
       - name: Set PostgreSQL versions
         id: set-versions
         run: |
@@ -57,6 +54,17 @@ jobs:
       - name: Checkout Repo
         uses: supabase/postgres/.github/actions/shared-checkout@HEAD
 
+      - name: Install nix
+        uses: cachix/install-nix-action@v27
+        with:
+          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=
+
+      - name: Fix nix permissions  
+        run: sudo chown -R $USER /nix/var/nix/profiles/per-user
+
       - id: args
         uses: mikefarah/yq@master
         with: