This PR changes the way we are reading values from `.env` to allow absent keys to pass `undefined` forward, and being ignored when generating gn args, as opposed to have our build commands providing a default in the absence of the key. This change will allow defaults to be stored in `.gn` files, rather than always being overwritten by our build command. Improper default overwritting is what caused the crash relating to these two PRs: https://github.com/brave/brave-core/pull/28463 https://github.com/brave/brave-core/pull/29891