Auto merge of #2853 - matklad:reduce-duplication, r=alexcrichton

Reduce duplication in `Context` creation

There was some duplicated code for `Context` creation in `cargo_clean` and `cargo_rustc`. I've tried to remove it by moving the common part into `Context::new`. Not sure that this is the right thing to do though, it's just something I came across while tracing `rustc_info` flow.

Additional possible refactoring would be to remove `Default` bound from `BuildConfig`.