ci: logs over 1mb are getting rejected by Loki

Created by: jhchabran

Example: https://buildkite.com/sourcegraph/sourcegraph/builds/150519#018114ed-f676-487a-b0ea-8f28752d2271/459-462

Slack context: https://sourcegraph.slack.com/archives/C07KZF47K/p1653917535527729?thread_ts=1653915784.426589&cid=C07KZF47K

Basically, some verbose jobs are racking up logs very quickly, which are then getting rejected by Loki for being too large. This is problematic, because it impedes our ability to search for previous failures.