ssh-agent: fix non-standard process check w/ pgrep

Confirmed to work on MacOS, OpenBSD, Solaris and busybox.

plugins/ssh-agent/ssh-agent.plugin.zsh

@@ -30,7 +30,7 @@ if [[ $_agent_forwarding == "yes" && -n "$SSH_AUTH_SOCK" ]]; then
 elif [[ -f "$_ssh_env_cache" ]]; then
 	# Source SSH settings, if applicable
 	. $_ssh_env_cache > /dev/null
-	ps -o cmd -p $SSH_AGENT_PID | grep -q ssh-agent || {
+	pgrep ssh-agent | grep -q $SSH_AGENT_PID || {
 		_start_agent
 	}
 else