Cargo skips "Doc-tests" when targeting x86_64-pc-windows-msvc.

[thoren-d]

Problem
If I run cargo test out of the box, "Doc-tests" is run and cargo invokes rustdoc as expected, but if I set --target x86_64-pc-windows-msvc on Windows (after installing the target with rustup), "Doc-tests"

Steps

1. cargo test --target i686-pc-windows-msvc
   "Doc-tests" in the output
2. cargo test --target x86_64-pc-windows-msvc
   "Doc-tests not in the output

Notes
i686-pc-windows-msvc is the default target used on Windows.

Output of cargo version: cargo 1.42.0 (86334295e 2020-01-31)
Output of rustup --version: rustup 1.21.1 (7832b2ebe 2019-12-20)
Output of rustc --version: rustc 1.42.0 (b8cedc004 2020-03-09)

[ehuss]

Cross-compilation of doc tests is currently an experimental feature. See #7040 for more detail.