build_sysext: Detect dev rebuild of release tag, fix build ID issue

[t-lo]

build_sysext uses a base squashfs (basically a full snapshot of the Flatcar OS image) to build custom sysexts on top. Before building it ensures the base image actualy matches the OS version in the repository root.

The version string includes a BUILD_ID which might be auto-generated (by including common.sh) if it is not present in the version file - e.g. when the version is an official release (tag). This build ID auto-generation causes issues with the version check when image build and sysext build scripts run independently - each will generate its own build ID, and this will cause build_sysext's version check to fail.

build_sysext will now use the build id from the base squashfs when it is not set in the source tree's version.txt to work around that issue. This is a more general solution than 361eda2 (which this patch reverts) as it directly addresses the issue in build_sysext instead of working around it in sysext_prod_builder.

How to use

• Check out this branch

  git clone https://github.com/flatcar/scripts.git
  cd scripts
  git checkout t-lo/fix-sysext-build-id-issue-in-image-to-vm

• Build it

  ./run_sdk_container -t
         ./build_packages
         ./build_image
         ./image_to_vm.sh --from ../build/images/amd64-usr/latest/ --format' 'qemu_uefi'
  

Testing done

Ran the above builds.

Cherry-pick / backport

To all active mainenance branches except LTS (which doesn't use sysext), i.e. flatcar-3975 flatcar-4012 flatcar-4054

[github-actions]

Build action triggered: https://github.com/flatcar/scripts/actions/runs/10613043126

[t-lo]

Tests are green and PR is approved, but main has moved on. Will force rebase and merge since it's a small change and well understood.

[t-lo]

Merged and cherry-picked for flatcar-3975, flatcar-4012, flatcar-4054.