How do I clamp the wire so it doesn't pull out of the box? I know that
there
are clips that can be put into place, but the sheathing of the garage wall
precludes my using them. I guess I could put the clip in backwards, but
they are wider than the pancake box is high.

Do I just leave the wire loose and depend on the "staple" inside the
garage to hold the wire?

Just want to be sure that this will meet code here in Calgary. Don't want
to fail my inspection.

The clamp is as much to protect against electrical fires in the box as to
hold the wire in place. A hole in a box is unacceptable.