The installer script seems to install a symlink to the nupst binary under /usr/bin/ but the service nupst creates tries to start the binary from /usr/local/bin/

You can't directly access the files (like install.sh for example) without being logged in anymore, because of that the download of the install.sh fails. Downloading the releases still works tough so maybe also export the installer as an asset to the latest release. Only viable workaround seems to be to clone the project via https (archive download doesnt work) and copy the install.sh to the machine where it should be installed.