Set BUNDLE_GEMFILE for generate-buildpacks-json
Set BUNDLE_GEMFILE for generate-buildpacks-json

Bundler 1.16.0 is causing issues when not set

Signed-off-by: Jackson Feeny <c36636f229e2e0887bbecb5e06d5626818aed501@gmail.com>

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci