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

[flexiondotorg]

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

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Name	Status	Preview	Comments	Updated (UTC)
zero-to-nix	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 14, 2023 9:22am