How to execute "systemctl --user" as a different user

Issue

• Trying to execute systemctl --user as a different user fails with Failed to connect to bus: No such file or directory

  # systemctl --user list-units
  Failed to connect to bus: No such file or directory
  

• Trying to execute systemctl --user after setting the environment variable XDG_RUNTIME_DIR for the target user fails with Failed to connect to bus: Operation not permitted

  # id -u user1
  1000
  # export XDG_RUNTIME_DIR=/run/user/1000
  # systemctl --user list-units
  Failed to connect to bus: Operation not permitted