Some commands don't work with CI builds but could

[jsha]

https://github.com/rust-lang/docs.rs/pull/1892/files#diff-b06b44e6583254ce69af8cedabc04a73a97599d593676e1bbad7ee442240ccf1R121-R128

When using a CI build, commands like this fail:

        let res = Command::new(&self.workspace, self.toolchain.rustc())
            .args(&["--version"])
            .log_output(false)
            .run_capture()?;

That's because rustwide tries to invoke:

$ rustc +6703d43029387eef125596dd7071d0d4fab902c3 --version
error: toolchain '6703d43029387eef125596dd7071d0d4fab902c3' is not installed

@jyn514 proposed we could solve this either by invoking the CI binary directly, or by running rustup toolchain link.