support xtask custom command alias

[sify21]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom image (optional)

Describe your request

The xtask workflow is described here: https://github.com/matklad/cargo-xtask

Following the guide, define an alias for xtask command in .cargo/config.toml

[alias]
taskA = "run --package xtask-taskA --"

I can't run cross taskA directly, it'll report that cross doesn't support the subcommand taskA and use cargo instead. I have to type the full command as cross run --package xtask-taskA

Describe why this would be a good inclusion for cross

For ease of use. A senario would be running rust-bindgen as a xtask to generate the binding code for paticular target.