Fix OTEL metrics exporter warnings in Dagger builds

The Dagger engine shim sets OTEL_METRICS_EXPORTER before our module
loads, so os.environ.setdefault was a no-op. Switch to a hard override.
Remove the redundant workflow-level env var since the fix belongs in
the module.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.forgejo/workflows/build-container.yaml

@@ -73,10 +73,6 @@ jobs:
     needs: detect
     if: needs.detect.outputs.dagger != '[]'
     runs-on: k8s
-    env:
-      # Disable Python SDK OTLP metrics exporter — the Dagger engine's local
-      # OTLP endpoint returns 500s, causing ~9s retry cycles per minute.
-      OTEL_METRICS_EXPORTER: none
     strategy:
       matrix:
         container: ${{ fromJson(needs.detect.outputs.dagger) }}