Add support for --opt run-variant-env=<json object> for passing
environment variables when running an image variant.

Sample use:

  buildctl build --frontend gateway.v0 \
  --opt source=$(LOCAL_BUILDKIT_IMAGE) \
  --local context=. --local dockerfile=. \
  --opt target=xxx \
  --opt run-variant=true \
  --opt run-variant-env='{"PIPELINE_RUNNING":"1"}'

Bug: T328054