[14elias]

Fixes #5144

Currently, rustup target add always pulls targets from the first installed channel toolchain alphabetically, ignoring any +toolchain override passed on the command line.

This causes a couple of issues:

• If you use a custom toolchain like rustup +stage1 target add , it still suggests targets even though custom toolchains don't have rustup-managed targets.
• If you specify a toolchain like +nightly, it might end up suggesting targets from another installed toolchain instead of nightly.

This changes fixes this by resolving the toolchain in this order:

1. Check if a +toolchain override was passed (if it's a custom toolchain, suggest nothing).
2. If not, check the active toolchain for the directory/default.
3. Fall back to the first installed toolchain if no active toolchain is set.