Bump DeterminateSystems/update-flake-lock from 19 to 20

[dependabot[bot]]

Bumps DeterminateSystems/update-flake-lock from 19 to 20.

Commits

• da2fd6f Update action.yml
• fd510d2 Merge pull request #78 from DeterminateSystems/DS-970
• 1a057b7 ci: clearly name steps in update.yml
• 51e1459 DS-970: Add Magic Nix Cache and other workflow changes
• 4fbf969 actions/checkout@v2 -> actions/checkout@v3
• 55ddfbe Bump actions/checkout in example
• See full diff in compare view

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