[convert] move 'stageName = ""' in covert after checking for circular dependency
[convert] move 'stageName = ""' in covert after checking for circular dependency

To have stageName in error output in case one stage depends on itself

Signed-off-by: Stepan Blyshchak <2e5bd4d72cb4af7b39a766a2e1d24cdd4fe8c25b@gmail.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc