pass extra information to build.nu for custom packages

[a-stevan]

related to

• fix the Rust toolchain FMotalleb/nu_plugin_clipboard#16

Description

because Nupm switches directories when installing a custom package and running build.nu, Cargo will use the system's version of itself instead of what's specified in rust-toolchain.toml.

this requires something like the following to make sure the version specified in rust-toolchain.toml is used:
https://github.com/FMotalleb/nu_plugin_clipboard/blob/00cc3d3e6a09df674c3a3fdbc1411c4937dde4b0/build.nu#L26-L32

i'm wondering if we could provide to build.nu some extra information in a new argument?

something like

def main [package_file: path = nupm.nuon, package_info: record<cargo_toolchain: string>] {
    cargo $"+($package_info.cargo_toolchain)" ...
}