Gitlab CI: Rework our tag usage again

Gitlab CI: Rework our tag usage again

Now that we've had jobs running on both amd64 and arm64 hosts for a
while, we have enough data to look at usage and findings. For the world
build job, make use of the new DEFAULT_FAST_TAG and only build it once,
on either amd64 or arm64 as we don't run in to host specific results
there. For sandbox, continue to build on both arm64 and amd64 hosts as
we can find host specific breakage that way. Remove the mistaken
restriction on sandbox64_lwip.

Signed-off-by: Tom Rini <trini@konsulko.com>