cmd/cue: default registry configuration file

[rogpeppe]

As of this change CUE_REGISTRY now supports a file-based configuration mode by setting it to file:....

If it's unset, it defaults to a central registry constant.

It would be useful to support a default configuration file (for example $CUE_CONFIG_DIR/registry.cue) that would be used when $CUE_REGISTRY is unset.