Downloading Nix dependencies is slow

[LightAndLight]

Example job: https://github.com/LightAndLight/ipso/actions/runs/3505790488/jobs/5872341175 (click the settings icon and show timestamps) (start, end). Total duration: 4.5 minutes.

Downloading the dependencies for nix develop .#tests takes ~40 seconds. That's ~15% of the total job duration.