Remove env var processing from secrets_args

Inside of the [secrets_args](https://github.com/bottlerocket-os/twoliter/blob/f6eba80d0568091daefeb3733e727fc239ddff20/tools/buildsys/src/builder.rs#L688) function, we read several variables from the environment instead of the centralized arguments in `args.rs`. As an optimization, we should remove these and leave the argument collection to `args.rs`.

Start in tools/buildsys/src/builder.rs at the secrets_args function referenced by the issue, then compare its environment-variable handling with the centralized argument collection in args.rs. Done means secrets_args no longer processes those environment variables and the argument collection remains centralized in args.rs.

Written by the indexing model from the issue text.