Set HOME if it's not defined in certbot-auto (#4503)

* set HOME if it's not defined

* use ~root rather than /root

letsencrypt-auto-source/letsencrypt-auto.template

@@ -15,6 +15,11 @@ set -e  # Work even if somebody does "sh thisscript.sh".
 
 # Note: you can set XDG_DATA_HOME or VENV_PATH before running this script,
 # if you want to change where the virtual environment will be installed
+
+# HOME might not be defined when being run through something like systemd
+if [ -z "$HOME" ]; then
+  HOME=~root
+fi
 if [ -z "$XDG_DATA_HOME" ]; then
   XDG_DATA_HOME=~/.local/share
 fi