bug#24168: Offloading builds does not honor --keep-failed

[ng0]

This bug is to keep track of and not forget to add this feature to
build offloading:

When I offload builds to my build machine(s), I want to keep the results
of failed builds to investigate the reason for failure.

Currently this does not happen with offloading, we should fix this as
its a nice feature to have.
-- 
♥Ⓐ  ng0
Current Keys: https://we.make.ritual.n0.is/ng0.txt
For non-prism friendly talk find me on http://www.psyced.org