The `IMAGE_UNDER_TEST` variable set in `.b2c-test` got broken with
the merge of 7d474c1 (ci: Move most stuff out of root .gitlab-ci.yml).
During the shuffling, the `MESA_BASE_TAG` and `MESA_IMAGE_TAG`
variables were dropped, leading to `IMAGE_UNDER_TEST` being an
unexisting container.
To make this issue less likely to happen in the future, this patch
drops the code duplication that led to `IMAGE_UNDER_TEST` to be
the same as `MESA_IMAGE` and instead re-uses .use-debian/x86_test-vk
to generate `MESA_IMAGE`, which we then use verbatim in
`IMAGE_UNDER_TEST`.
The renaming is `MESA_IMAGE` into `IMAGE_UNDER_TEST` there to make the
distinction clear between the image run by gitlab-runner (what is
usually called `MESA_IMAGE` but we instead hardcode to valve-infra's
trigger container), and the image we are running on the test machines.
Fixes: 7d474c1 (ci: Move most stuff out of root .gitlab-ci.yml)
Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Reviewed-by: Charlie Turner <cturner@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15555>
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