Have nix-build trigger regeneration of repo.nix files.

[cessationoftime]

I think it may be possible to have the genNix commands triggered by nix-build and only as needed. If we get a hash representing our list of dependencies and write it to a file then we could check that hash and automatically run commands to regenerate repo.nix files as needed. Even better would be a way to diff the old dependencies and new ones and just update for the changes. Though we may want to switch to sbt 1.0 first.

[nightkr]

There is no guarantee that the Nix builder will have access to the .nix files, and it won't if you're running in multi-user mode.

[nightkr]

The project has moved to GitLab: https://gitlab.com/teozkr/Sbtix/merge_requests/32