fix(ci): move linux onto the larger blacksmith runner

What:
- move the Linux CI job to blacksmith-16vcpu-ubuntu-2404
- switch Linux Nix installation to cachix/install-nix-action

Why:
- the recovery line exhausted disk on ubuntu-latest during the Linux aggregator build
- the larger runner kept the normal parallel build graph intact and the cachix installer worked reliably there

Tests:
- final green CI run on main: 24217118174

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ concurrency:
 
 jobs:
   linux:
-    runs-on: ubuntu-latest
+    runs-on: blacksmith-16vcpu-ubuntu-2404
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -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