experimental: add built-in Incus sysext image

[tormath1]

TBD

How to use

---
variant: flatcar
version: 1.1.0
storage:
  files:
    - path: /etc/flatcar/enabled-sysext.conf
      contents:
        inline: |
          incus
    - path: /etc/userdb/core:incus-admin.membership
      contents:
        inline: "\n"
    - path: /etc/subuid
      append:
        - inline: |
            root:1001000000:1000000
    - path: /etc/subgid
      append:
        - inline: |
            root:1001000000:1000000

Testing done

TBD

• [ ] CI: http://jenkins.infra.kinvolk.io:8080/job/container/job/packages_all_arches/4060/cldsv/
• [ ] Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• [ ] Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.
• [ ] Inspect USE flags
• [ ] Add new packages to .github/workflows/portage-stable-packages-list
• [x] rebase on https://github.com/flatcar/scripts/pull/1742
• [x] review downstream patch (tc-is-cross-compiler) + wait for Go1.21 upgrade (https://github.com/gentoo/gentoo/pull/36323)

Related to: https://github.com/flatcar/Flatcar/issues/1319

[tormath1]

First try:

# incus launch images:ubuntu/22.04 ubuntu-01
# incus start ubuntu-01
# incus exec ubuntu cat /etc/os-release
PRETTY_NAME="Ubuntu 22.04.4 LTS"
NAME="Ubuntu"
VERSION_ID="22.04"
VERSION="22.04.4 LTS (Jammy Jellyfish)"
VERSION_CODENAME=jammy
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU_CODENAME=jammy

NOTE:

• Socket is not starting correctly because it can't find the group incus-admin
• Same as podman, we need to generate subuid / subgid

[github-actions[bot]]

Build action triggered: https://github.com/flatcar/scripts/actions/runs/9052482849

[pothos]

Same as podman, we need to generate subuid / subgid

Would be good to use the same approach as done there to make sure that the extensions don't conflict.