report_image_diff: rework variables extraction

This MR is:

• an alternative solution for proposal from SECO in [YT-148] CI: rework image_diff job: Move to generated pipeline part.
• is a replacement for the old commit, which solves the same problem: 4fd7f8a8

The old solution made too much assumptions about the pipeline and needed to re-implement some of routine in charge of the GitLab YAML interpretation. To get rid of this fragile logic, SECO proposed to move the report-image-diff job to the generated part of pipeline. But as difficulties appeared with the implementation of this proposal (see details in YT-148), I decided to implement another solution and to extract MACHINE and DISTRO variables from the jobs themselves:

• MACHINE is part of the build job name (build-$MACHINE);
• DISTRO is printed by bitbake and can be found in build log.