docs: update readme with Determinate Nix Installer

[przemyslaw-wlodek]

This PR includes new instructions about installing Nix. The Determinate Nix Installer is much more user-friendly and stable.

Review Checklist

Basics

• PR assigned to the PR author(s)
• input-output-hk/daedalus-dev and input-output-hk/daedalus-qa assigned as PR reviewers
• If there are UI changes, Alexander Rukin assigned as an additional reviewer
• PR has appropriate labels (release-vNext, feature/bug/chore, WIP)
• PR link is added to a Jira ticket, ticket moved to In Review
• PR is updated to the most recent version of the target branch (and there are no conflicts)
• PR has a good description that summarizes all changes
• PR contains screenshots (in case of UI changes)
• CHANGELOG entry has been added to the top of the appropriate section (Features, Fixes, Chores) and is linked to the correct PR on GitHub
• There are no missing translations (running yarn manage:translations produces no changes)
• Text changes are proofread and approved (Jane Wild / Amy Reeve)
• Japanese text changes are proofread and approved (Junko Oda)
• Storybook works and no stories are broken (yarn storybook)
• In case of dependency changes yarn.lock file is updated

Code Quality

• Important parts of the code are properly commented and documented
• Code is properly typed with typescript types
• React components are split-up enough to avoid unnecessary re-renderings
• Any code that only works in main process is neatly separated from components

Testing

• New feature/change is covered by acceptance tests
• New feature/change is manually tested and approved by QA team
• All existing acceptance tests are still up-to-date
• New feature/change is covered by Daedalus Testing scenario
• All existing Daedalus Testing scenarios are still up-to-date

After Review

• Update Slack QA thread by marking it with a green checkmark

[michalrus]

Maybe suggest that the user sets our binary cache from the very beginning? Then it's a single-command setup.

So the command would be:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | \
  sh -s -- install \
  --extra-conf "substituters = https://cache.nixos.org/ https://cache.iog.io/" \
  --extra-conf "trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ="

[michalrus]

@przemyslaw-wlodek wants to keep the rest of the instruction intact (Slack).