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 https://github.com/bitcoindevkit/bdk/pull/1165#discussion_r1374071091)
  • 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 https://github.com/bitcoindevkit/bdk/pull/1165#discussion_r1374078143

  
  - 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 https://github.com/bitcoindevkit/bdk/pull/1165#issuecomment-1782308478

• [ ] Use cargo-nextest. https://github.com/bitcoindevkit/bdk/pull/1165#issuecomment-1782278992

[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.