ignore `rust-toolchain` file in crates building in sandboxed env

[yaahc]

From a comment in rust-lang/rust#75180 about a regression not being caught in artichoke by crater.

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@yaahc @Dylan-DPC I pulled the rustops/crates-build-env container and Artichoke builds successfully in it, but I did notice that the rustup-installed cargo initiated an install of the toolchain specified in Artichoke's rust-toolchain file. I'm guessing this type of network activity is what the crater sandbox prevents.

Given that crater's intent is to build projects with specific versions of rustc, I think it makes sense to ignore rust-toolchain files in the repository if they are present.

Originally posted by @lopopolo in rust-lang/rust#75180 (comment)

[pietroalbini]

Looking into this. It might actually be the cause of all those "error"s.

[pietroalbini]

Ok, opened #561 with a fix (bringing in rust-lang/rustwide#52).

Turns out the error was not rust-toolchain, as Rustwide bypasses that already (the file does not matter when you call cargo +toolchain). The problem was rustwide removing all path dependencies from the Cargo.toml, thus breaking the build.

[pietroalbini]

This should be fixed now!