test_external_oqsprovider: Use working checkout of liboqs

Fixes #18898

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/18899)

(cherry picked from commit 7e32ca79)