packaging: Updating guix.scm to GPLv3+, setting source to checkout.

I talked to Jan Nieuwenhuizen, who agreed to license their
contributions to this file under GPLv3+, simplifying the licensing.
The rest of 8sync is still LGPLv3+... this just applies to the Guix
packaging.  We also borrow some code from guile-sdl2 to set our source
to be from the package itself, making testing the package from
guix.scm easier.

* guix.scm: Switch license to GPLv3+.
Switch source to be from the local directory, borrowed from
guile-sdl2.