Add support for the `--update-nix' flag when building Nix

This commit adds support for the `--update-nix' flag for the build-webkit script
in a similar fashion than the EFL port.

Now we can build Nix with `Tools/Script/build-webkit --nix --update-nix',
which will first run jhbuild to update the dependencies, if needed,
before proceeding to building Nix itself.

Reviewed by: Marcelo Lira