dgit: Once again cope with archive skew

dgit: Once again cope with archive skew

Since our http rearrangements, we have lost the ability to try
multiple dsc versions which means we cope badly with archive skew.

This was an editing error: in 7821907696db
   dgit: Abolish url_get in favour of url_fetch
we omitted to notice that url_get implicitly does what url_fetch needs
Ok404 to do.  So we didn't add the Ok404 parameter.

Fix this by adding it now.

Closes: #935874
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>