Support Buildpacks API 0.8: switch from build/detect args to env variables

[dmikusa]

• When using buildpack API 0.8 or greater, the platform directory and build plan path are provided as environment variables to buildpack bin/detect, in addition to the deprecated positional arguments (Set env vars for positional args in build and detect lifecycle#818)
• When using buildpack API 0.8 or greater, the platform directory, buildpack plan path, and buildpack layers directory are provided as environment variables to buildpack bin/build, in addition to the deprecated positional arguments (Set env vars for positional args in build and detect lifecycle#818)