Support the JSONNET_PATH env variable

[tmeijn]

Found this while give this tool a little test: the env var is not picked up. Also tried SJSONNET_PATH but that did not work either.

[He-Pin]

yeah, seems it's not been implemented yet.
https://github.com/google/jsonnet/blob/599a949dc973e8dd11aa9c3ebc923bc964751671/cmd/jsonnet.cpp#L82