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. Bugfix on 0.4.8.1-alpha.

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