Build worker ffb-dock-01 out of disk space for 3 days - destroying packages builds

[Hannu Nyman]

I am not sure who is monitoring the buildbots these days, but the build 
worker ffb-dock-01 has now been three days out of disk space, destroying most 
of the packages builds.

https://buildbot.openwrt.org/main/packages/#/

https://buildbot.openwrt.org/main/packages/#/grid

https://buildbot.openwrt.org/main/packages/#/workers/24

File 
"/opt/venv/lib/python3.9/site-packages/buildbot_worker/commands/transfer.py", 
line 388, in finished
self.fp.close()
builtins.OSError: [Errno 28] No space left on device