Turns the anchor derivation into something that can actually be
built (a call creating a propagated build inputs file), and builds it.
This should fix the anchoring logic we have on canon.
Change-Id: If6a7662b82e2e396388980f65e332cf67a45b46e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4763
Tested-by: BuildkiteCI
Autosubmit: tazjin <mail@tazj.in>
Reviewed-by: sterni <sternenseemann@systemli.org>