geos: use fetchFromGitHub for src access

[imincik]

geos is currently using

  src = fetchurl {
    url = "https://download.osgeo.org/geos/${pname}-${version}.tar.bz2";
    sha256 = "sha256-eauMq/SqhgTRYVV7UuPk2EV1rNwNCMsJqz96rvpNhYo=";
  };