(Nice to have) Implement pushing of intermediate images containing the `envbuilder` binary for later resumption

[mafredri]

The purpose of this feature is to improve workspace build/startup speed even when some image layers have changed.

As shown in a PoC: coder/envbuilder#185 (comment), it's possible to create and push intermediate images during the build process. However, doing so breaks the final image.

Investigate if this is still an issue and/or if we can work around it and create the intermediate images without affecting the final image. (For instance, our refactor of creating "reproducible" images in Kaniko might have improved the situation.)

A tip: Look into cloning and restoring the build args before/after creating the image ((*stageBuilder).args.Clone()).