blubber buildkit frontend: Support --opt run-variant-env
Add support for --opt run-variant-env= 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