fix(ci): use cachix installer on blacksmith linux

The blacksmith Linux runner change exposed a new infra issue: the
Determinate installer action times out during download on the self-hosted
runner before Nix is available.

Use cachix/install-nix-action for the Linux job so the larger runner can
actually execute the Nix build.

Tests:
- verified remote failure in CI run 24212145536: nix-installer-action timed out during Install Nix on blacksmith

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
         run: scripts/check-flake-lock-owners.sh
 
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v13
+        uses: cachix/install-nix-action@v31
 
       - name: Run Linux CI aggregator
         run: nix build .#checks.x86_64-linux.ci --accept-flake-config