ci(kb): disable ES disk watermark in functional tests

The KB functional tests intermittently failed in setup-kibana.cjs with
6 minutes of `failed to retrieve password hash for reserved user [elastic]`
errors. The underlying cause is in the ES container logs:

  UnavailableShardsException: at least one primary shard for the index
  [.security-7] is unavailable

When the Buildkite host disk is above the default 85% low watermark,
ES refuses to allocate the security index's primary shard, so the
reserved-user store never becomes readable and every `elastic` auth
attempt fails. Intermittent because it depends on the agent's disk
state at run time.

For a single-node trial test cluster on ephemeral CI storage, the
disk allocator buys nothing, so turn it off.

Refs #416

Author: MattDevy

.buildkite/run-kb-tests.sh

@@ -85,6 +85,7 @@ docker run \
   --env "ELASTIC_PASSWORD=${ES_PASSWORD}" \
   --env "xpack.security.http.ssl.enabled=false" \
   --env "xpack.security.transport.ssl.enabled=false" \
+  --env "cluster.routing.allocation.disk.threshold_enabled=false" \
   --env "ES_JAVA_OPTS=-Xms512m -Xmx512m" \
   --detach \
   --rm \