Re-add Nix magic cache (#1851)

Author: palfrey

.github/actions/prepare-nix/action.yaml

@@ -48,5 +48,9 @@ runs:
         fi
 
     - name: Install Nix
-      uses: >- # v17
+      uses: >- # https://github.com/DeterminateSystems/nix-installer-action/releases/tag/v17
         DeterminateSystems/nix-installer-action@21a544727d0c62386e78b4befe52d19ad12692e3
+
+    - name: Add Nix magic cache
+      uses: >- # https://github.com/DeterminateSystems/magic-nix-cache-action/releases/tag/v11
+        DeterminateSystems/magic-nix-cache-action@def9f5a5c6a6b8751c0534e8813a5d0ad2635660