Set CFG_VERSION in src/cargo/ops/cargo_rustc/mod.rs

When I use `cargo build` to build cargo then I get
```
       Fresh hammer v0.1.0 (https://github.com/wycats/hammer.rs#ref=c085c639)
       Fresh hamcrest v0.1.0 (https://github.com/carllerche/hamcrest-rust.git#ref=05acf768)
       Fresh toml v0.1.0 (https://github.com/alexcrichton/toml-rs#ref=a3c7f2c3)
   Compiling cargo v0.0.1-pre (file:/home/niels/workspace/my_cargo)
src/bin/cargo-version.rs:32:5: 32:41 error: environment variable `CFG_VERSION` not defined
src/bin/cargo-version.rs:32     println!("{}", env!("CFG_VERSION"));
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
Build failed, waiting for other jobs to finish...
```
Here is a quickfix to save the build.

NB: Alternatively it might be prettier to use CARGO_PKG_VERSION_MAJOR and friends in cargo-version.