Update debian build. python 3.8 -> 3.12

[hmaerki]

Linux python 3.8 -> 3.12

This PR focuses mainly on improving the linux build

If this PR is considered to be pulled, I could split and document every commit in a more granular way.

linxu: python 3.8 -> 3.12

• This is done by:
  • Docker image ubuntu 18.04 -> 24.04
  • Replace hardcoded /usr/lib/python3.8/ by dynamic reference.
• Benefits for the user:
  • Python warnings about missing ctypes dissapeared
  • Python 3.12
  • Appimage tested on ubuntu 2025.10 and 2026.04

Allow forking the repo by

• hardcodeed sigrokproject -> ${{ github.repository_owner }}

New github actions which create docker images for

• New actions:
  • .github/workflows/docker-sigrok-appimage.yml
  • .github/workflows/docker-sigrok-mxe.yml
• These actons use GITHUB_TOKEN. A individual package token is not required anymore
• These scripts are obsolte/redundant now:
  • ci/appimage/sigrok-appimage-create-appimage.sh
  • docker-sigrok-mxe.sh

Added two commits from

• @danielkucera