`main_binary` should reuse the caller's target

[epage]

I suspect when running cargo test --target TRIPLET that main_binary will still use the default TRIPLE.

[epage]

We can workaround this by having a build.rs capture the target triplet environment variable.

Example: https://github.com/mmstick/cargo-deb/blob/master/build.rs

[epage]

The responsibility for this is moving to assert_cmd and tis will be addressed in assert-rs/assert_cmd#4

assert_cli is going to morph into a wider-focused tool, including things like assert_fs. See #41