Revert "Revert "goaidentity: Fix deadlock in goaalarm on_cancelled""

This reverts commit 7436669dde08abd5d4c6fe4716f43263b3567161.
goaidentity: use proper main context

The previous commit uses the wrong main context, which ultimately
leads symptoms including crashes and 100% cpu usage.

This commit just tweaks the previous commit a little bit, to use
the proper context.