Revert "Try adding magic nix cache to nix CI build."

This reverts commit 295c6b0485.

This was actually slower than what we had before (14m on second
run, as compared to 12m).

.github/workflows/ci.yml

@@ -144,10 +144,9 @@ jobs:
       fail-fast: true
     steps:
     - uses: actions/checkout@v3
-    - name: Install Nix
-      uses: DeterminateSystems/nix-installer-action@v4
-    - name: Run the Magic Nix Cache
-      uses: DeterminateSystems/magic-nix-cache-action@v1
+    - uses: cachix/install-nix-action@v22
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
     - run: |
         nix-shell --run "cabal update && cabal test --disable-optimization"