[SPIKE] can we include a status update as part of a system rebuild?

[gmodena]

Suggested in https://discourse.nixos.org/t/managing-flatpaks-declaratively/34532/13.

[gmodena]

Display the line by line update for each flatpak that is updated by piping the update outputs from the flatpak update command to the NixOS rebuild command

This turned out to be a bit more convoluted than I had anticipated. Under the hood flatpak update is executed from a systemd unit, triggered on activation by systemct. By default, systemctl swallows the command output and redirects it to journald. While there's option to intercept and redirect stdout, I have not yet found a decent solution that works for both HM and nixos.