Fix wrong timestamp when importing packages with rhnpush (bsc#1235970)

[parlt91]

What does this PR change?

When using rhnpush the last_modified column would get an incorrect timestamp when using timezones other than UTC on the server.

Remove calculation of the (wrong) timestamp from headerSource.populate() and rely on the trigger creating the timestamp when no value to last modified is provided.

rhn_package_mod_trig_fun()

if tg_op='UPDATE' then
  if new.last_modified = old.last_modified or
     new.last_modified is null then
       new.last_modified := current_timestamp;
  end if;
else
  if new.last_modified is null then
       new.last_modified := current_timestamp; <------
  end if;
end if;
new.modified := current_timestamp;

Tested ISSv1 not affected by this change (using different code to import packages).

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: BUGFIX

• DONE

Test coverage

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• DONE

Links

Issue(s): https://github.com/SUSE/spacewalk/issues/26231

• DONE

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[meaksh]

LGTM! BTW, I noticed the wrong value for last_modified was also happening for packages that were synced by reposync, and also fixed by this PR.