Rustup should know why a toolchain is installed and detect garbage toolchains

Problem you are trying to solve

a full rust toolchain is quite large, around 2GB. this puts the user in a tricky situation, where they don't want to have a bunch of extra toolchains installed, but they also don't want to accidentally delete a toolchain used by a project the use a lot, and then have to redownload it over slow wifi

Solution you'd like

either as part of the --verbose flag, or as a new flag (like list --why), rustup would display the reason that a specific toolchain was installed. if it was installed because of a rust-toolchain file, the path to that file is listed. toolchains installed before this update was made would have their reason listed as "unknown"

Notes

a possible improvment would be for rustup to track what rust-toolchain files reference a particular toolchain, although that would add significant complexity

Start by tracing the rustup list --verbose flow and how rust-toolchain files cause toolchains to be installed. Define the recorded reasons, the "unknown" result for existing installations, and how the displayed path or reason indicates that the requested behavior is complete.