ci: rust-cache was caching nothing, it needs workspaces - #3

busbar has no ref '3/merge', so this build used busbar@dev instead. On a pull_request, `github.ref_name` is '<number>/merge' rather than a branch name, which is the usual cause - pass `busbar_ref: \main` from the caller to say what you meant. Falling back is deliberate: an unresolvable sibling ref used to fail the checkout with an unreadable git error and no mention of the ref, which read like the plugin's own tests were broken.