Only upload builder artifacts from workflow runs triggered after merge

[James-Mart]

Changes

• Artifacts generated during PRs will no longer be uploaded to ghcr.io, and will instead only be uploaded as local artifacts to the github action.
• Workflows are significantly more intelligent now. For example, a workflow has the logic to know whether it needs to download one of its dependency images from a local github action artifact, or whether it should pull from ghcr.io. And if it pulls from ghcr.io, it knows how to look up the latest image (not using a latest tag).
• The builder workflows were combined into one, parameterized by the desired base version of ubuntu.
• A single dispatcher workflow is the only thing automatically triggered on PR or merge, and it is responsible for determining what other workflows to trigger (and how to parameterize them) based on the changed files.

Notes on centralizing dispatching

There were lots of complexities trying to manage each of the separate workflows and trigger them at the correct times. For example, if files were changes that would cause the psibase_contributor image to be rebuilt, then I should trigger that workflow. But I should not trigger it if files were also changed that would trigger a rebuild of the 2204 builder image, since that is a dependency of the psibase_contributor and therefore psibase_contributor would need to run afterwards. Building such logic like this into a workflow file using normal path exclusion doesn't work the way I expected, and I decided that the most straightforward way to accomplish our complicated build strategies here is to simply extract all the logic into a central _dispatcher.yml workflow that figures everything out and dispatches the workflows appropriately.