etc/diskspace: factor out keepBuilds: number of pkg build versions to keep
etc/diskspace: factor out keepBuilds: number of pkg build versions to keep

fpco/stackage,Daniel-Diaz/stackage,mrkkrp/stackage,michalkonecny/stackage,simonmichael/stackage,clinty/stackage,iand675/stackage,jonschoning/stackage,massysett/stackage,soenkehahn/stackage