ci: Move ARM rootfses to stable

We build in Debian buster but were currently testing in bullseye-based
ramdisks. This has started being a problem since Python 3.7 was removed
from bullseye.

[ Also bumped arm_test containers, by anholt ]

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5433>

.gitlab-ci/container/lava_arm.sh

@@ -78,7 +78,7 @@ debootstrap \
     --variant=minbase \
     --arch=${DEBIAN_ARCH} \
      --components main,contrib,non-free \
-    testing \
+    buster \
     /lava-files/rootfs-${DEBIAN_ARCH}/ \
     http://deb.debian.org/debian