fix(workflows): fix the rechunking step to work when toggled

[Atmois]

Rechunker now successfully runs when uncommenting its steps in the build workflow. This was achieved by implementing the following changes:

• Rework the build to use the root storage of the builder to not require copying the image (which is not possible for large images due to size constraints even when maximising build space). The stage is adapted from ublue-os/bazzite's build stage.
• Reworking the push to GHCR and auth stage to work for both rechunked and normal images without needing any changes.

The following was also done to improve UX when working with the rechunker:

• Added new comments to instruct the user what to do more clearly to use the rechunker successfully
• Simplified enabling the rechunker to just be uncommenting the stage, this before required tweaking to work out the box
• Maximise build space stage appears to be redundant as in all testing for this PR not once (when before all rechunked images required it) was it required to be enabled. It will be left in for backwards compatibility or if someone really maxes out the builder and needs it.

The new workflow was tested with the following images both with and without the rechunker. All attempts were successfully with no modifications needed to anything beyond implementing the new workflow:

• ublue-os/bazzite:stable
• ublue-os/aurora:stable
• ublue-os/bluefin:stable
• ublue-os/base-main:latest
• fedora/fedora-bootc:43
• https://github.com/Atmois/atmo-bazzite
• https://github.com/noelmiller/mordor