Make `rustup update` completely update the default target before working on others

Problem you are trying to solve

For anyone who has multiple targets installed, completing a rustup update can take quite a while. It seems the order of operations is:

1. Download rust-std for all installed targets
2. Download cargo, rust-src, and other components
3. Download rust-std for the native/default target (?)
4. Download rustc
5. Remove old components
6. Install downloaded components

However, often times only the default target is of immediate interest.

Solution you'd like

Instead, do something like the following order:

1. Move the old toolchain directory to a different name
2. Download and install rustc
3. Download and install rust-std for the default toolchain/target
4. Download and install components
5. Download and install all remaining targets
6. Remove the backup directory

This should make it feasible to start working with the most commonly used target within a couple of minutes while everything else updates in the background.

Notes

No response

Start by tracing the existing rustup update flow and its current download, installation, backup, and cleanup order. Compare that flow with the proposed default-target-first sequence, then verify that the default target becomes usable before other targets finish and that remaining components still update and cleanup completes successfully.