Buildx will stall when buildkit is configured with OTel but the OTel endpoint isn't available

Reproduction steps:

* Start buildkit with `OTEL_EXPORTER_OTLP_ENDPOINT` set to something (or otherwise enable OTLP tracing in some way).
* Have that endpoint not exist (service is not running(.
* Run buildx to perform a build.

The build will stall at the end for approximately 20 seconds before it eventually exits.