Automatically copy all local `source` files

[thomaseizinger]

Currently, one has to duplicate all local files that are included in the package as part of assets. However, the action already knows which files should be present: It is specified in the source array.

PKGBUILD files are valid shell scripts. Wouldn't it make more sense to do source PKGBUILD and then iterate over source and copy everything that doesn't start with http://?

[KSXGitHub]

If your PKGBUILD was generated, you can have one source of truth for both assets and PKGBUILD#source.

I also welcome PR to implement new feature, as long as it doesn't break backward-compatibility and error-prone.