Suggest `x build library` for a custom toolchain that fails to load `core`

[jyn514]

Code

x build std --target x86_64-unknown-linux-gnu
cd ../example
cargo +stage1 build --target i686-unknown-linux-gnu

Current output

error[E0463]: can't find crate for `core`
  = help: consider downloading the target with `rustup target add i686-unknown-linux-gnu`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

Desired output

error[E0463]: can't find crate for `core`
  = help: consider adding the standard library to the sysroot with `x build library --target i686-unknown-linux-gnu`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

Rationale and extra context

rustup doesn't manage targets and components for custom toolchains. We can detect in rustc that this is a custom toolchain by checking env!("CFG_RELEASE_CHANNEL") == "dev" and changing the help output if so.

Other cases

No response

Anything else?

cc https://discord.com/channels/442252698964721669/1124482842051747860

[Noratrieb]

This had to be reverted because it broke UI tests with channel=dev. #113288