dirmngr: Fix Let's Encrypt certificate chain validation.

* dirmngr/certcache.c (find_cert_bysubject): Return the first trusted
certififcate if any.

This is basically the same as using OpenSSL with ist
X509_V_FLAG_TRUSTED_FIRST flag. See
https://www.openssl.org/blog/blog/2021/09/13/LetsEncryptRootCertExpire/

• GnuPG-bug-id: T5639