Switch the Checkov step to soft-fail (#30)

Until we can move to using a pinned Checkov image version – the lack of image
pinning is causing the build step to break unpredictably.

Author: ferozsalam

.buildkite/pipeline.yaml

@@ -17,3 +17,4 @@ steps:
   - label: ":lock: security - checkov"
     command: .buildkite/ci-checkov.sh    
     agents: { queue: standard }    
+    soft_fail: true