mailcheck: fix problem when calling check_cmd with a variable in profile

See issue #1 on original project.

My $MBLAZE is set to /Users/x/.config/mail

In my mblaze profile, I customized MailCheckCommand to be:
MailCheckCommand: fdm -f $MBLAZE/fdmrc -q fetch

My small fix permits this otherwise, mailcheck aborts.

mailcheck

@@ -43,7 +43,7 @@ wakeup () {
 
 mailcheck () {
 	echo $(ts) checking ...
-	${check_cmd}
+	$(echo $check_cmd | envsubst)
 	if [ -n "${index_cmd}" ]; then
 		echo $(ts) indexing ...
 		${index_cmd}