Delay setting the sig algs until after the cert_cb has been called

Otherwise the sig algs are reset if SSL_set_SSL_CTX() gets called.

Fixes #7244

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/7257)

(cherry picked from commit 524006dd1b80c1a86a20119ad988666a80d8d8f5)