`rustup run $toolchain cargo` succeeds even if $toolchain is not installed

[kamalmarhubi]

This is unexpected, and can lead to confusion if trying out cargo build or cargo test with some older versions. You end up silently building against nightly instead (see below).

$ rustup --version
rustup 0.6.4 (e6dc1c6 2016-10-24)
$ rustup run 1.0 rustc --version
error: toolchain '1.0' is not installed
$ rustup run 1.0 cargo --version
cargo 0.13.0-nightly (717adc8 2016-11-02)

The call to cargo should fail as well.

To demonstrate that other non-cargo commands do not run:

$ rustup run 1.0 which cargo
error: toolchain '1.0' is not installed

It seems that the silent successful cargo commands use nightly:

$ cd $(mktemp -d)
$ cargo new bad  # to get a valid Cargo.toml
     Created library `bad` project
$ cd bad
$ rustup run 1.0 cargo rustc does-not-exist.rs -- --version
   Compiling bad v0.1.0 (file:///tmp/tmp.iiZg2tF5CK/bad)
rustc 1.14.0-nightly (5665bdf3e 2016-11-02)
    Finished debug [unoptimized + debuginfo] target(s) in 0.2 secs

I think it's using the nightly channel and not a specific nightly, as I hadn't installed 2016-11-02 explicitly:

$ ls $HOME/.multirust/toolchains
1.2.0-x86_64-unknown-linux-gnu  nightly-2016-10-26
1.8.0-x86_64-unknown-linux-gnu  nightly-2016-10-28
beta-x86_64-unknown-linux-gnu   nightly-x86_64-unknown-linux-gnu
nightly-2016-10-09              stable-x86_64-unknown-linux-gnu
nightly-2016-10-19

[Diggsey]

This seems to be fixed when I try to reproduce.