We need update kernel often. We need test kernel changes often.
Introduced `KERNEL_EXTERNAL_TAG` to differ between `KERNEL_TAG` which is
also used to rebuild the containers. We don't need rebuild containers
for the external kernel, so this way we don't have to.
Updating kernel goes wruuuuuum.
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23563>
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