ci: automatically mirror HEAD to trunk (#1405)

This change introduces a pipeline job called `mirror` that will push to
`refs/heads/trunk` automatically, in order to keep it in sync with the
default tree.

This job is restricted to the SoT for git-bug, and will only execute for
the first run of the workflow.

Refs: #1404
Change-Id: If65f83ce9058dd01aa74f6841dc58ac040284b18