Provide a documented way to use `cargo auditable` as a drop-in replacement for `cargo`

[Shnatsel]

This came up in the pull request about enabling cargo auditable in Nix by defaut.

There are two hurdles here:

1. cargo auditable calls cargo internally quite a lot. We need to make sure it doesn't recurse.
2. Some targets are currently not supported, notably WebAssembly, and cargo auditable will error out in this case. This is not an option for a drop-in replacement for cargo - we need to print a warning and keep going.

[Shnatsel]

Cargo sets only one environment variable when calling subcommands, and that's a path to itself in the CARGO env var.

Just following that convention (and letting people set it to the real cargo when using as a drop-in replacement) sounds good. I'm not going to require that variable to be set because I'm not sure if that would break anything (especially exotic things like sccache).

I think if I add another env var for ignoring unsupported platforms, we should be good. So something like this would work, if placed in $PATH and called cargo:

#!/bin/sh

export CARGO='/path/to/real/cargo'
export CARGO_AUDITABLE_IGNORE_UNSUPPORTED=true
cargo-auditable auditable "$@"

btw cargo-auditable auditable is not a typo, that's how Cargo calls subcommands and we actually take advantage of it.

@figsoda @zowoq would this work for Nix?

I could write this as a Rust executable as well, but I think a shell script is way easier to understand and debug - a compiled Rust binary would just be a black box. Transparency is key when you override default behavior.

[figsoda]

Didn't test anything out, but this should work for nix. I think a shell script in nixpkgs is good enough, no rust needed here.

[Shnatsel]

Alright, I'll implement support for these two environment variables then!

[Shnatsel]

As of commit 00030f0 the CARGO variable is implemented, so you can use the above snippet.

The CARGO_AUDITABLE_IGNORE_UNSUPPORTED isn't implemented yet, but shouldn't be a blocker for initial testing.

[Shnatsel]

Support for CARGO_AUDITABLE_IGNORE_UNSUPPORTED has landed in 74a2d3a

[Shnatsel]

Actually you can just use alias cargo="cargo auditable" without going through all this trouble. I've documented it in the README.

[Shnatsel]

I'm open to feedback on whether continuing the build but printing a warning when targeting an unsupported platform should just be the default.

Maybe a configuration option is overkill, since this behavior is basically never desirable.

[figsoda]

I am not sure about the semver aspect of this, but it (warning instead of error) definitely sounds like a saner default to me

[Shnatsel]

I'll ship it as 0.6 just to be on the safe side