Adapt wire-cell-toolkit recipe to depend on `libjsonnet` virtual instead of `+cppjsonnet` variant

[brettviren]

aaafacf adds a libjsonnet virtual that is provided by either the jsonnet or gojsonnet packages. Both provide the C API/ABI (and C++) library with the same name. gojsonnet still provides libgojsonnet. With this change to the go-jsonnet package, the wire-cell-toolkit recipe can be simplified to remove the cppjsonnet variant.