espup install should suceed even if the Xtensa Rust toolchain is already installed

[SergioGasquez]

Bug description

At the moment, if $RUSTUP_HOME/.rustup/toolchain/esp is present, the installation will fail with:

Error: espup::toolchain::rust::xtensa_rust_already_installed

  × ⛔  Previous installation of Rust Toolchain exists in: '/home/esp/.rustup/toolchains/esp'. Please, remove the directory before new installation.

espup should be idempotent and running several times espup install should lead to the same output (successful). Running several times espup install could also be a way of re-generating the export file, so it could be used as a solution for #61.

To Reproduce

Steps to reproduce the behavior:

1. espup install
2. espup install