PLUGIN: gpg-agent: export SSH_* environment variables too

If using the gpg-agent with --enable-ssh-support, the SSH_AUTH_SOCK and
SSH_AGENT_PID environment variables need to be exported once sourced
from GPG_ENV.  Otherwise, we get no benefit from the persisting these
values to GPG_ENV; subsequent openned terminals don't see the existent
gpg-agent as a process for an SSH daemon.

plugins/gpg-agent/gpg-agent.plugin.zsh

@@ -20,6 +20,8 @@ if ! gpg-connect-agent --quiet /bye > /dev/null 2> /dev/null; then
     if [ -f "${GPG_ENV}" ]; then
         . ${GPG_ENV} > /dev/null
         export GPG_AGENT_INFO
+        export SSH_AUTH_SOCK
+        export SSH_AGENT_PID
     fi
 
     # check again if another agent is running using the newly sourced settings