root

This commits adds support for parsing the `--manifest-path` option to cross. So
far, the `Cargo.toml` manifest of the crate (or its Cargo workspace) to compile
has been assumed to be in the current working directory. This means, that
relative crate dependencies were not supported, because those paths were not
mapped into the docker container.

Take the following example structure, where `my-bin` depends on `my-lib`:
.
├── my-bin
│   ├── Cargo.toml
│   └── src
│       └── main.rs
└── my-lib
    ├── Cargo.toml
    └── src
        └── lib.rs

This commits enables such scenarios, by running cross from `.` like so:
`cross build --manifest-path=my-lib/Cargo.toml --target x86_64-pc-windows-gnu`,
as `.` is mapped as the container's root, and the options passed through to
Cargo.

Related cross-rs#388 cross-rs#139 cross-rs#277 cross-rs#78

Co-authored-by: Kviring Alexey <alex@kviring.com>

Ensure we build the entire workspace using the `--workspace` flag both with and without the manifest path, and also ensure we build the binary with all our workspace dependencies.

`"${@}"` is an unbound variable on Bash 3.2, while `"$@"` is not, and Bash 3.2 is the pre-installed version on macOS.