Auto merge of #4630 - ehuss:new-test-temp-dir, r=alexcrichton
Auto merge of #4630 - ehuss:new-test-temp-dir, r=alexcrichton

Fix `new` tests on Windows inadvertently picking up ~/.cargo/config.

Fixes #4601.  By relying on `__CARGO_TEST_ROOT`, using a temp directory is no
longer necessary.

achanda/cargo,tshepang/cargo,eminence/cargo,rustyhorde/cargo,achanda/cargo,achanda/cargo,vojtechkral/cargo,tsurai/cargo,tsurai/cargo,tsurai/cargo,achanda/cargo,vojtechkral/cargo,rustyhorde/cargo,tsurai/cargo,tshepang/cargo,rust-lang/cargo,rust-lang/cargo,tshepang/cargo,rust-lang/cargo,rustyhorde/cargo,vojtechkral/cargo,tshepang/cargo,nipunn1313/cargo,nipunn1313/cargo,vojtechkral/cargo,rust-lang/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,rustyhorde/cargo,eminence/cargo,eminence/cargo,nipunn1313/cargo