Add AUR installed package to pacman cache

[falmp]

I'd like to have the AUR installed package installed on the pacman cache.

This could easily be done by copying the .tar.xz file to CacheDir option (default is /var/cache/pacman/pkg/). If the package is installed by using pacman -U file:///path/to/package.tar.xz, it'll be automatically added to the cache, but then it depends on RemoteFileSigLevel option, which is Required by default. So this solution would need some workaround.