Add a facility to rustup to show how its variables are currently set.

Problem you are trying to solve

Currently there is (or I can find) no way to see how the "variables" in rustup are set. The rustup set command allows you to set them but it has no --list variant neither does rustup show have a --variables variant. To be slightly more explicit, rustup set help displays:

Alter rustup settings

Usage: rustup[EXE] set <COMMAND>

Commands:
  default-host      The triple used to identify toolchains when not specified
  profile           The default components installed with a toolchain
  auto-self-update  The rustup auto self update mode
  auto-install      The auto toolchain install mode
  help              Print this message or the help of the given subcommand(s)

Options:
  -h, --help  Print help

But I cannot display how default-host, profile, auto-self-update or auto-install are configured.

Solution you'd like

I have no preference as to whether this is a new variant of show or set or both or even some other solution. If we take Unix shell command set as the exemplar, then rustup set would show this rather than assuming "help" as it does now, but since that is a breaking change it may well not be the preferred solution.

Notes

I have discussed this on zulip and been advised to raise an issue.