DS-970: Add Magic Nix Cache and other workflow changes

An assortment of GitHub Workflow changes, potentially including:

- Enable DeterminateSystems/magic-nix-cache-action@main
- Reference all DeterminateSystems actions via @main
- Make update.yaml consistent across repos
- Remove unnecessary github-token: from nix-installer-action
- Update actions/checkout@v2 to actions/checkout@v3

.github/workflows/ci.yml

@@ -13,5 +13,7 @@ jobs:
           fetch-depth: 0
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@main
+      - name: Enable magic Nix cache
+        uses: DeterminateSystems/magic-nix-cache-action@main
       - name: Shellcheck
         run: nix-shell --run 'shellcheck $(find . -type f -name "*.sh" -executable)'