commit2jigdo: Query objsize from correct repo when building objid map

I have hit multiple times the bug with `bare-user` repos:
ostreedev/ostree#1297

(Need to rebuild my dev container rather than relying on an updated
 `sudo make install` from ostree git)

Anyways if things are out of sync, this bug would manifest as the
main repo wouldn't have the object.

Closes: #1135
Approved by: jlebon

src/app/rpmostree-ex-builtin-commit2jigdo.c

@@ -540,7 +540,7 @@ build_objid_map_for_package (RpmOstreeCommit2JigdoContext *self,
        * "content-identical objects" i.e. they differ only in metadata.
        */
       guint32 objsize;
-      if (!query_objsize_assert_32bit (self->repo, checksum, &objsize, error))
+      if (!query_objsize_assert_32bit (self->pkgcache_repo, checksum, &objsize, error))
         return FALSE;
       if (objsize >= BIG_OBJ_SIZE)
         {