This adds the following (pre-existing) attributes to the jobset response:

- nrtotal
- lastcheckedtime
- starttime
- checkinterval
- triggertime
- fetcherrormsg
- errortime

Extend the jobset API response

This attribute allows to know if an error occurred or not: when an
error occurs, errormsg is not an empty string. Note we can not use the
errormsg attribute because it can be arbitrarily long and is excluded
from the jobset API response.

Add error boolean attribute to jobset API response

This removes a supper annoying set of messages that polute the logs:

    Aug 30 09:00:30 xxx.compute.internal hydra-server[957]: Trouble trying to detect your terminal size, looking at $ENV{COLUMNS}
    Aug 30 09:00:30 xxx.compute.internal hydra-server[957]: Term::Size::Any is not installed, can't autodetect terminal column width

Add Term::Size::Any

Only set buildMachinesFiles when nix.buildMachines is defined

It contains Hydra, PostgreSQL and an frontend proxy. So you can get a
running Hydra instance by doing

  $ nixos-container create hydra --flake hydra
  $ nixos-container start hydra

The web interface is available on port 80.