buildctl ignores/fails silently when ref is not specified in the export/import registry cache syntax

[amrmahdi]

When using registry cache, if a user incorrectly specifies something like --export-cache=type=registry,name=foo:bar buildctl will silently ignore the registry cache without displaying any warning/error message (the right syntax is ref=foo:bar).

Looking at the code it seems as an artifact from supporting the legacy cache import/export syntax which as documented in code should have been removed a long time ago.