update container version to v31

This version adds musl-tools package
which is required for rust-vmm/vhost#218
to compile userfaultfd crate for musl target.

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>

.buildkite/autogenerate_pipeline.py

@@ -60,7 +60,7 @@
 
 # This represents the version of the rust-vmm-container used
 # for running the tests.
-CONTAINER_VERSION = "v30"
+CONTAINER_VERSION = "v31"
 # This represents the version of the Buildkite Docker plugin.
 DOCKER_PLUGIN_VERSION = "v5.3.0"