ci: Nix enhancements

[realeinherjar]

Some enhancements were suggested in #1165.
They are non-blockers and I am moving them to a separate issue for the near future.

• Move to rustshop/flakebox

  • It can build derivations in parallel with pkgs.linkFarmFromDrvs (example, from ci: move to Nix #1165 (comment))
  • It will also use mold by default as the Linux linker. It can speed up builds.

• Upload failed derivations as artifacts.
  From fedimint/fedimint in ci: move to Nix #1165 (comment)

  - name: Prepare failed test build dirs
    if: always()
    run:  |
      set -x
      # delete source and target artifacts as it is huge and rather useless now
      sudo rm -Rf /tmp/nix-build-*/source || true
      # chown so actions/upload-artifact can access it
      sudo chown -R $USER /tmp/nix-build-* || true
      # delete unix sockets, as actions/upload-artifact can't handle them
      find /tmp/nix-build-* -type s -print0 | xargs -0 rm || true
  
   - name: Upload failed test build dirs
     if: always()
     uses: actions/upload-artifact@v3
     with:
       name: "fedimint-failed-test-logs-${{ github.run_number }}-${{ matrix.host}}"
       path: |
         /tmp/nix-build-*/
  

• Auto-correct typos.
  This also involves having a just setup.
  Example from https://github.com/fedimint/fedimint/blob/0ef5039086a4cd863484cbfacd638d8a0d42c233/justfile#L80, in ci: move to Nix #1165 (comment)

• Use cargo-nextest. ci: move to Nix #1165 (comment)

[notmandatory]

I put these in the beta.0 release for now, but may move them to future milestone if not urgent / ready to work on now.