Rebuilding all the test containers for a change that only affects the
build containers is costly and unnecessary. Split the `DEBIAN_BASE_TAG`
into `DEBIAN_BUILD_BASE_TAG` and `DEBIAN_TEST_BASE_TAG` to make it
possible to rebuild only the relevant half of the containers.
Signed-off-by: Valentine Burley <valentine.burley@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36736>