v8: use 4.5.107 by default

Default v8 was fixed at 3.30.33, so running nix-build nixpkgs -A v8
would give us version 3.30.33, whereas nix-env -i v8 would install
version 4.5.107. Now, both commands build 4.5.107, and there is still an
option to install version 3.30.33