SCP-4726 Migrate to nix flakes

[hrajchert]

This PR includes changes from PR-19 and unblocks PR-16, which could be reviewed at the same time.

It migrates the nix code to flakes, removing the update materialized and updating the marlowe-cardano dependency.

Deployment configuration is not included in this PR.

Pre-submit checklist:

• Branch
  • [ ] Tests are provided (if possible)
  • [X] Commit sequence broadly makes sense
  • [X] Key commits have useful messages
  • [X] Relevant tickets are mentioned in commit messages
  • [X] Formatting, materialized Nix files, PNG optimization, etc. are updated
• PR
  • [X] Self-reviewed the diff
  • [X] Useful pull request description
  • [X] Reviewer requested