chore: use magic-nix-cache instead of cachix

This removes one dependency of the project, by moving the Nix cache
store into Github. There is no expected speed up, except for lower
latency between the actions runner and the cache store.

.github/workflows/nix.yml

@@ -18,13 +18,10 @@ jobs:
           ssh-key: ${{ secrets.WORKFLOW_SSH_KEY }}
 
       - name: Install Nix
-        uses: cachix/install-nix-action@v20
+        uses: DeterminateSystems/nix-installer-action@v4
 
-      - name: Setup Cachix
-        uses: cachix/cachix-action@v12
-        with:
-          name: aiken-lang
-          authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
+      - name: Run the Magic Nix Cache
+        uses: DeterminateSystems/magic-nix-cache-action@v1
 
       - name: Run cargo2nix
         shell: bash