deprecate config file

[yosifkit]

Along with #114, we should deprecate the config file since we mostly just use the environment variables anyway (e.g., https://github.com/docker-library/meta-scripts/blob/da908ae960aaf23ff67b1d465af0ab1b47183fd8/sources.sh#L28).

Environment variables are easy enough to permanently add to things like bashrc (and Jenkins config) that we don't really need the config file (and actively don't consult it in meta scripts anyway because that's increased complexity).

We need to make sure we don't rely on it anywhere (like me locally 🙈😢).