CI: Switch to Cachix for Nix caching. (#133)

### What

We are mostly not using our cache, because GitHub's own cache
rate-limits us to the point where it's unusable.

Cachix should do better.

### How

Replace the Magic Nix Cache action with the Cachix action, using our
newly-minted auth token.

.github/workflows/benchmarks.yaml

@@ -11,15 +11,20 @@ jobs:
     name: Benchmark
     runs-on: ubuntu-latest
     steps:
-      - name: Install Nix ❄
-        uses: DeterminateSystems/nix-installer-action@v6
-
-      - name: Run the Magic Nix Cache 🔌
-        uses: DeterminateSystems/magic-nix-cache-action@v2
-
       - name: Checkout 🛎️
         uses: actions/checkout@v4
 
+      - name: Install Nix ❄
+        uses: cachix/install-nix-action@v22
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Set up the Nix Cache 🔌
+        uses: cachix/cachix-action@v12
+        with:
+          name: hasura-v3-dev
+          authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
+
       - name: Build the Docker image 🔨
         run: |
           docker load < $(nix build --no-link --print-out-paths '.#ndc-postgres-docker')

.github/workflows/build-images-and-deploy.yaml

@@ -26,10 +26,15 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install Nix ❄
-        uses: DeterminateSystems/nix-installer-action@v6
+        uses: cachix/install-nix-action@v22
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Run the Magic Nix Cache 🔌
-        uses: DeterminateSystems/magic-nix-cache-action@v2
+      - name: Set up the Nix Cache 🔌
+        uses: cachix/cachix-action@v12
+        with:
+          name: hasura-v3-dev
+          authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
 
       - id: gcloud-auth
         name: Authenticate to Google Cloud 🔑