That workflow file runs npm pack on a branch, resulting in a .tgz file of the project.
Because JavaScript, HTML and CSS are not compiled there’s not normally a need to run this step on a fork.
Also, ghost install installs .zip files, and this workflow generates a .tgz file. It’s not even clear that it generates something that be installed directly.
If you want to maintain some differences, I recommend you create a file in the “unified diff” file format and save it as a .diff or .patch file.
Then after you run the standard upgrade process, run something like patch <my-patch.diff to patch the code with your differences.