Replace downloaded edk2 uefi shell by those provided in edk2-shell

As discussed on the arch-releng mailing list, we want to have a reproducible way of providing a UEFI shell on the created ISO. Currently the .efi binaries are downloaded from the edk2 upstream.

While adding checksums for the downloaded files is a way to ensure they are at least the same, it is better to be able to reproducibly build them from scratch. This has been achieved with edk2-shell.

It's time to replace the curl calls with cp calls and add edk2-shell to archiso's depends for the next release.