d/gpg-agent.gpg-agent-ssh.user.socket: Check enable-ssh-support

Check whether the user has set enable-ssh-support in their ~/.gnupg/gpg-agent.conf before setting the SSH_AUTH_SOCK environment variable. This prevents gpg-agent from stomping on ssh-agent from openssh-client, which some users need for security key support (ecdsa-sk or ed25519-sk keys).

Closes: #1078787