XDG path support for rustup

Historically, rustup has relied on $RUSTUP_HOME (defaulting to $HOME/.rustup) and $CARGO_HOME (defaulting to $HOME/.cargo) as monolithic directories for configuration, state, and data. In coordination with the Cargo team, this proposal aims to migrate `rustup` to use the standardized XDG Base Directory Specification on Linux. To achieve this safely, we will transition from a "dual-directory" approach to a "fine-grained" directory approach, unblock Cargo's own XDG migration, and establish a strict backward-compatibility protocol for existing installations.