fix(rustup-mode): improve clap error format

[rami3l]

Closes #3905 by preventing clap errors from surfacing upwards in rustup_mode::main.

Before

> rustup update self
error: error: invalid value 'self' for '[toolchain]...': invalid toolchain name: 'self'

For more information, try '--help'.
: invalid toolchain name: 'self'

After

> RUSTUP_FORCE_ARG0=rustup rustup run stable cargo run -- update self
[..]
error: invalid value 'self' for '[TOOLCHAIN]...': invalid toolchain name: 'self'

For more information, try '--help'.