XDG_RUNTIME_DIR is not set and /run/user/<UID> does not exist when using su or sudo to switch user

Issue

• su - <user> or sudo -u <user> -i is used to temporarily run commands as <user>.
• The XDG_RUNTIME_DIR and other XDG-related environment variables are not set.
• The /run/user/uid directory is not created.
• Problem happens even after adding pam_systemd.so to /etc/pam.d/system-auth, as recommended in solution 5592491.