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

Author: flexiondotorg

.github/workflows/update.yml

@@ -1,14 +1,16 @@
 name: update-flake-lock
 on:
   workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * 0'
 
 jobs:
   lockfile:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
-      - name: Checkout repository
-        uses: actions/checkout@v2
-      - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v1
       - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@v4
+        uses: actions/checkout@v3
+      - uses: DeterminateSystems/nix-installer-action@main
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - uses: DeterminateSystems/flake-checker-action@main
+      - uses: DeterminateSystems/update-flake-lock@main