Reduce amount of build logs kept

[tedinski]

Jenkins is using 3+ GB in /usr/lib/jenkins which is a bit much. It looks like it's just because we get logs for every build forever stored.

I think we could set up the archiving properties to discard older build logs for most repos. We really just want to preserve old ones for Silver itself. And maybe slightly longer for AbleC/Copper, but I'm not sure we need those older logs as much. But as far as I can tell, there's no good reason to bother with old build logs for the extensions.

[ericvanwyk]

Do we even use old logs for anything? It seems fine to get rid of lots of these to me.