ci: Append build image tag to LAVA tag used for minio path

ci: Append build image tag to LAVA tag used for minio path

This automatically ensures the LAVA kernel / device-tree / rootfs files
are updated along with the build image, so any relevant changes in the
latter are reflected in the former.

v2:
* Use the same definition of DISTRIBUTION_TAG in .lava-test:amd64 as in
  kernel+rootfs_amd64
v3:
* Extend comment about $MESA_BASE_TAG in .lava-test:amd64 (Eric Anholt)

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7988>