cargoSnapshot: 2015-04-02 -> 2015-06-17, hide from nix-env

Update snapshot to avoid rust-lang/cargo#976, which otherwise breaks the
build.

Also move the `cargoSnapshot` derivation inside a set in
pkgs/top-level/all-packages.nix in order to hide the `cargo-snapshot`
packages from `nix-env -qa`, since it's only used to build the `cargo`
package.