since the .{src,bin}pkgs stuff was merged, our build system is leaving
.{start,end}.timestamp files around, which are useless in most cases.
anonym has complained about it, so here's a branch that only generates
these files when building from Jenkins.
This means that we can't trivially generate .{src,bin}pkgs files
outside of Jenkins anymore. If someone is lacking this feature some
day, tell me and I'll revisit.
