Appveyor is broken

[jdm]

This is a result of #21809 but I don't understand why it's only affecting windows.

rustup -V
rustup 1.12.0 (573895abc 2018-07-07)
mach rustc --version
rustc 1.30.0-nightly (7061b2775 2018-08-28)
mach cargo --version
error: Found argument '--manifest-path' which wasn't expected, or isn't valid in this context
	Did you mean to put '--manifest-path' after the subcommand 'bench'?
USAGE:
    cargo.exe --version
For more information try --help
Command exited with code 1

[SimonSapin]

Maybe we don’t run mach cargo --version on other CI systems? Replacing that with cargo --version (without mach) should be fine now that we use rustup with a rust-toolchain file.