fix: scripts (preinstall/postinstall) not working for `flat`

[rickymohk]

The --scripts should go to pkgbuild in order for preinstall and postinstall script to work. So the single productbuild command has to be separated into pkgbuild and productbuild.

I am not sure if the --scripts in productbuild has any practical use. Might need to add an new option if need to retain the --scripts for both productbuild and pkgbuild