Test that project works as cargo extension

[eopb]

Make sure that after installing cargo-override it can be run as a cargo extension, as cargo override

[eopb]

This is the bug I expected to see

cargo override --help
error: unexpected argument 'override' found

Usage: cargo-override --path <PATH>

For more information, try '--help'.

We need to accept an initial override arg

[eopb]

Running cargo override results in a call to cargo-override override so we need to support this subcommand