Add 64 bit and lite builds

[m-roberts]

This PR adds 64-bit OS build support, in addition to adding support for building a "pi-topOS Lite" version.

Notes

All packages that required packaging changes will need to be released and promoted through the production repos for non-experimental builds to work properly.

pi-topOS Lite/64-bit Changes

• Installs qemu-user-static >v5 to allow arm64 chroot
• OS artifacts now include their build type (so that each one has a unique name in the matrix strategy - otherwise they all are added into 1 zip)
• Generalise QEMU steps (e.g. use qemu-*-static instead of qemu-arm-static)
• Separate out desktop patches from system patches
• Add conditional checks for steps that only run on particular OS configurations
• OS builds will now remove non-release apt sources by default
  • i.e. only able to update from 'release pi-topOS' repo after build
  • e.g. further experimental packages aren't installed during user upgrades
  • makes it so that early builds mimic final build behaviour, and it is easy for a developer to re-add the source for more updates

Other changes

• Fix GitHub artifact uploading to prevent "zip in a zip"
• Standardise build pipelines and share environment via dotenv file

Suggested improvements for pi-topOS Lite

Pending Features

• [Request] Add support for headless use-cases pi-top-Firmware-Updater#72 - support for managing devices headlessly.

Size improvements

• [Request] Remove OpenCV requirement from the core for 300MB pi-top-Python-SDK#582 - reduce install size of core SDK.
• [Request] Loosen CLI package dependencies for 50MB pi-top-Python-SDK#583 - reduce install size of pt-miniscreen.

[m-roberts]

@jcapona keep_preview_source is still lurking around

[jcapona]

thanks @m-roberts! 🙌🏼