ci: compress LAVA rootfs with zstd instead of gzip

Visible size reduction and minor performance improvement at no cost.

rootfs measure:

```
2022-07-27 11:15:16.235268: 475MB downloaded in 111.59s (4.26MB/s)
2022-07-27 15:07:40.984857: 425MB downloaded in 85.57s (4.97MB/s)
```

So let say approx. 95s vs 85s if we assume 5MB/s, which can bring us
10s speedup...

Acked-by: Guilherme Gallo <guilherme.gallo@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17776>

.gitlab-ci/container/lava_build.sh

@@ -112,7 +112,8 @@ apt-get install -y --no-remove \
                    python3-numpy \
                    python3-serial \
                    unzip \
-                   wget
+                   wget \
+                   zstd
 
 
 if [[ "$DEBIAN_ARCH" = "armhf" ]]; then
@@ -238,14 +239,14 @@ fi
 
 du -ah /lava-files/rootfs-${DEBIAN_ARCH} | sort -h | tail -100
 pushd /lava-files/rootfs-${DEBIAN_ARCH}
-  tar czf /lava-files/lava-rootfs.tgz .
+  tar --zstd -cf /lava-files/lava-rootfs.tar.zst .
 popd
 
 . .gitlab-ci/container/container_post_build.sh
 
 ############### Upload the files!
 ci-fairy minio login --token-file "${CI_JOB_JWT_FILE}"
-FILES_TO_UPLOAD="lava-rootfs.tgz \
+FILES_TO_UPLOAD="lava-rootfs.tar.zst \
                  $KERNEL_IMAGE_NAME"
 
 if [[ -n $DEVICE_TREES ]]; then