`rustup show` doesn't correctly report installed targets for custom toolchains

[rami3l]

Following @wesleywiser's report in #4248, I realized that custom toolchains with optional components is a use case that needs more attention.

Minimal reproduction below:

> rustup toolchain install nightly-2023-09-30
[..]

> mv $RUSTUP_HOME/toolchains/nightly-2023-09-30-aarch64-apple-darwin $RUSTUP_HOME/toolchains/stuff
[..]

> rustup +stuff show --verbose
[..]
active toolchain
----------------
name: stuff
active because: overridden by +toolchain on the command line
compiler: rustc 1.74.0-nightly (8ce4540bd 2023-09-29)
path: [$RUSTUP_HOME]/toolchains/stuff
installed targets: