Force-reinstall a toolchain

Describe the problem you are trying to solve

My computer is haunted, and if I don't set the right environment variables before rustup update, the toolchain it installs will fail to use any procedural macros with fun errors like

error: /nix/store/90illc73xfs933d06daq6d41njs8yh66-glibc-2.32-37/lib/libc.so.6: version `GLIBC_2.33' not found (required by /home/jonas/dev/nrf-hal/target/debug/deps/libcortex_m_rt_macros-67ec7e8a841acd06.so)
   --> /home/jonas/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-rt-0.6.13/src/lib.rs:403:1
    |
403 | extern crate cortex_m_rt_macros as macros;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Describe the solution you'd like

Because I re-learn this fun fact after every Rust release, I end up with broken toolchains a lot. It would be helpful if rustup supported a way to force a reinstallation of a toolchain and all of its components and targets, without having to use rustup uninstall, since that would mean I have to reinstall every component and target I use.

Start by tracing the rustup update and rustup uninstall command paths. Determine how a force-reinstall operation could preserve and reinstall the toolchain's existing components and targets; done means users can reinstall the toolchain without manually reinstalling each component or target.