Strengthen build-arch rebuild capabilities

[jlebon]

Right now, if build-arch fails, one can mostly rerun it with the same parameters. There is one gotcha: if the failure happens at any point after the Archive step, the artifacts for the given build ID and arch will already exist.

We should strengthen the job so that it handles this window better.

[jlebon]

One simple solution @dustymabe and I we came up with is to check upfront if the artifacts already exist, and if so just skip straight to the Sign Images step.

[jlebon]

A temporary hack on this is to have a salvage-build job that we can run manually that just fetches the artifacts of a build, signs them, emits the compose repo import, and schedules the follo-wup kola jobs.

[prestist]

This might be a bit complicated, for little gain; revisit with @jlebon; This #566 (comment) seems the most relevant.

[jschintag]

One option is to have the steps that notifies the ostree repo to be a warning instead of a failure, saving us from having to run cloud and update tests manually for the build.