shared/logs-show: do not overwrite journal time in export format with source timestamps

shared/logs-show: do not overwrite journal time in export format with source timestamps

Using _SOURCE_{MONOTONIC,REALTIME}_TIMESTAMP in place of the results of
sd_journal_get_{monotonic,realtime}_usecs in export formats might cause
internal inconsistency of realtime timestamp values within a journal export,
violating the export file format and causing systemd-journal-remote to
mass-generate journal files.

Fix this by using the real journal timestamps for
__{REALTIME,MONOTONIC}_TIMESTAMP.