Package script should take pre-gen'd JSON from draft, not make a new one

[earlephilhower]

The GHCI publish script should not regenerate the ZIP and JSON file before uploading it to the server because the sha256sum of the newly generated ZIP file will not match the one in the release (because of timestamp changes for the source files).

Instead, the package script should just pull the generated JSON file from the release and push it to the boards URL w/o adjustment.