Make networkstatus_getinfo_by_purpose report published_on again.

When we implemented prop275 in 0.4.8.1-alpha, we changed the behavior of networkstatus_getinfo_helper_single to omit meaningful published_on times, replacing them with "2038-01-01". This is necessary when we're formatting a routerstatus with no additional info, since routerstatus objects no longer include a published_on.

But in networkstatus_getinfo_by_purpose, we do have a routerinfo that does have a published_on. This patch uses that information to report published_on times in our output when we're making a "virtual" networkstatus for a big file of routerinfo_t objects. This is mostly important for bridge authorities, since when they dump a secret list of the bridges, they want to include published_on times.

Closes #40855 (closed). Bugfix on 0.4.8.1-alpha.