build-snap: leaves `remote-build` artifacts on LP if cancelled

The `remote-build` action does not clean up after itself if cancelled.

It could do some cleanup on start (completed / failed builds and the corresponding code), or maybe we need periodical cleanup.

Start by inspecting the `remote-build` action and tracing what happens when it is cancelled. Clarify whether cleanup should happen on action start or periodically, including completed and failed builds and their corresponding code. Done means cancelled runs no longer leave `remote-build` artifacts on Launchpad.

Written by the indexing model from the issue text.