livefs builds can hang forever

I've recently been seeing a few livefs builds hanging. The current ones look like:

```
Running germinate...
Downloading http://ftpmaster.internal/ubuntu/dists/hirsute/main/binary-armhf/Packages.xz file ...
Decompressing http://ftpmaster.internal/ubuntu/dists/hirsute/main/binary-armhf/Packages.xz file ...
Downloading http://ftpmaster.internal/ubuntu/dists/hirsute/main/source/Sources.xz file ...
Decompressing http://ftpmaster.internal/ubuntu/dists/hirsute/main/source/Sources.xz file ...
Downloading http://ftpmaster.internal/ubuntu/dists/hirsute/restricted/binary-armhf/Packages.xz file ...
Decompressing http://ftpmaster.internal/ubuntu/dists/hirsute/restricted/binary-armhf/Packages.xz file ...
Downloading http://ftpmaster.internal/ubuntu/dists/hirsute/restricted/source/Sources.xz file ...
Decompressing http://ftpmaster.internal/ubuntu/dists/hirsute/restricted/source/Sources.xz file ...
* Downloading http://archive-team.internal/seeds/ubuntu.hirsute/STRUCTURE
```

(the hanging forever is probably a germinate bug; I will file that separately)

.deb package builds have a watchdog that causes them to time out after a period of inactivity (no build log output). It would be good to have the same for livefs builds, so that we don't end up eternally waiting for builds that are never going to complete.