Cargo fallback for toolchain linked toolchains seems to not work on Windows

[petrochenkov]

Reproduction should look something like

# Build my own rustc
../x.py build --stage 1

# Link rustup to it
rustup toolchain link dev build/x86_64-pc-windows-gnu/stage1
rustup default dev

# Check rustc and cargo, rustc works, but cargo doesn't
rustc --version
rustc 1.20.0-dev (f75d16a48 2017-07-07)
cargo --version
error: toolchain 'dev' does not have the binary `cargo.exe`

# Switch back to nightly toolchain, cargo is available again
rustup default nightly-x86_64-pc-windows-gnu
cargo 0.21.0-nightly (eb6cf012a 2017-07-02)

I believe I've seen this working previously, but maybe it was on Linux, so I'm not sure.

[petrochenkov]

Triage: this is still an issue.

[petrochenkov]

The problem disappeared after uninstalling rustup and then installing the fresh version again from scratch.
So apparently installation of older rustup versions could become broken in some way leading to cargo fallback being unavailable.

[petrochenkov]

Hmm, the difference with reinstalled rustup is that the .rustup\fallback\cargo.exe appeared, it wasn't there before.