Update on demand for the git cache on build server

[bartoszbetka]

Resolves #191

[cameel]

Please put it in a separate block. It makes it easier to see that you're switching users.

[bartoszbetka]

This require also add second block for task below.

[cameel]

Yes.

[cameel]

You should not have to run it as root. The whole idea with setuid was to allow normal users to run it.

EDIT: After finishing the review I see that systemctl does not let use do what we wanted to achieve. We need to use sudo after all. See my comment in the issue that explains the problem and shows what we want to do instead: #191 (comment)

[bartoszbetka]

I know, we agreed to use systemctl with sudo. We only run sudo with systemctl that is less dangerous than when we run all scripts with sudo. The systemd service control what is execute.

[cameel]

I thought we agreed to use systemctl to avoid sudo (which is more problematic because it requires messing with /etc/sudoers/) and to use sudo as a last resort if systemd does not have any feature that let's you define who can run a service. It doesn't so let's go with sudo. I don't see any benefit in having a service if you need to use sudo anyway.