tarball: fix the job

Find-tarballs was failing on some `requireFile` that was fixed-output
but didn't specify `outputHashMode`. Apparently, nix is fine with that
and uses the "flat" default, although that's undocumented.

maintainers/scripts/find-tarballs.nix

@@ -18,7 +18,8 @@ let
 
   fetchurlDependencies =
     filter
-      (drv: drv.outputHash or "" != "" && drv.outputHashMode == "flat" && drv.postFetch or "" == "" && drv ? urls)
+      (drv: drv.outputHash or "" != "" && drv.outputHashMode or "flat" == "flat"
+          && drv.postFetch or "" == "" && drv ? urls)
       dependencies;
 
   dependencies = map (x: x.value) (genericClosure {