`mach cargo COMMAND` will run `cargo-COMMAND` using `cargo build`
arguments by default. However, it is possible to tune the arguments
using either command-line arguments, or a YAML file in `config/cargo/`.
A file `config/cargo/template.yaml` can be used to create a new
configuration for a new cargo sub-command.
Differential Revision: https://phabricator.services.mozilla.com/D166780