sulogin-shell: avoid heredocs

They require a writable /tmp dir (in the bash implementation).
Let's use echo, and not 'echo -e' since that doesn't seem to be completely
portable.

Fixes #6052.

src/sulogin-shell/systemd-sulogin-shell.in

@@ -4,11 +4,9 @@ if [ -x /bin/plymouth ]; then
     /bin/plymouth --wait quit
 fi
 
-cat <<EOF
-You are in $1 mode. After logging in, type "journalctl -xb" to view
-system logs, "systemctl reboot" to reboot, "systemctl default" or ^D to
-boot into default mode.
-EOF
+echo "You are in $1 mode. After logging in, type \"journalctl -xb\" to view"
+echo "system logs, \"systemctl reboot\" to reboot, \"systemctl default\" or ^D to boot"
+echo "into default mode."
 
 @SULOGIN@
 @SYSTEMCTL@ --job-mode=fail --no-block default