Release process: drop the "Build the almost-final images" step

This is a time-sink in the release process: it can take up to 45 minutes and it comes right after an opportunity to take a break (./bin/prepare-included-website-for-release).

AFAICT we do this step for 2 reasons:

• generate the $ALMOST_FINAL_BUILD_MANIFEST
• have an image we can use to update the SquashFS file order

In both cases, we could use artifacts from Jenkins builds of the release branch:

• For bugfix releases it should be just fine to take the latest completed build.
• For RCs, we need to wait for a build that started after we completed the "Freeze" section.