fetcher: Lazily create tmp directory

The tmp directory is lazily created for each fetcher instance, since
it may require superuser permissions and some instances only need
_ostree_fetcher_request_uri_to_membuf() which keeps everything in
memory buffers.

src/libostree/ostree-repo.c

@@ -419,9 +419,7 @@ _ostree_repo_remote_new_fetcher (OstreeRepo  *self,
   if (tls_permissive)
     fetcher_flags |= OSTREE_FETCHER_FLAGS_TLS_PERMISSIVE;
 
-  fetcher = _ostree_fetcher_new (self->tmp_dir_fd, fetcher_flags, NULL, error);
-  if (fetcher == NULL)
-    goto out;
+  fetcher = _ostree_fetcher_new (self->tmp_dir_fd, fetcher_flags);
 
   {
     g_autofree char *tls_client_cert_path = NULL;