New build-plan tests failing when compiled with rust

[ehuss]

When attempting to update Cargo in the rust repo, the following tests failed:

    build_plan::cargo_build_plan_build_script
    build_plan::cargo_build_plan_simple
    build_plan::cargo_build_plan_single_dep

Detailed output can be found here: https://ci.appveyor.com/project/rust-lang/rust/build/1.0.7409/job/s65ul0p5xdw9420o

Essentially the problem is that in the rust tree, cargo tests are run with CARGO_TARGET_DIR set, and the above tests do not take that into consideration.

cc @mshal