cargo-audit build fails with cr117 toolchain

[rillian]

Description

Since merging the chromium 117 changes, ci builds fail compiling cargo-audit.

FAILED: cargo_audit/release/cargo-audit 
19:13:58  python3 ../../build/gn_run_binary.py /home/ubuntu/workspace/brave-browser-build-noplatform-nightly/src/third_party/rust-toolchain/bin/cargo build --release --manifest-path /home/ubuntu/workspace/brave-browser-build-noplatform-nightly/src/brave/tools/crates/cargo_audit/v0_17/crate/Cargo.toml --target-dir /home/ubuntu/workspace/brave-browser-build-noplatform-nightly/src/out/Component_audit/cargo_audit
[...]
19:13:58  error[E0635]: unknown feature `proc_macro_span_shrink`
19:13:58    --> /home/ubuntu/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.56/src/lib.rs:92:30
19:13:58     |
19:13:58  92 |     feature(proc_macro_span, proc_macro_span_shrink)
19:13:58     |                              ^^^^^^^^^^^^^^^^^^^^^^
19:13:58  

The offending feature is enabled by proc-macro2 v1.0.56 on nightly builds. The upstream chromium rust toolchain build we're using here is compiled as a nightly release, so the likely conflict is between unstable features.

Steps to Reproduce

1. rustup install nightly-2023-07-21
2. cd brave/tools/crates/cargo_audit/v0_17/crate
3. cargo +nightly-2023-07-21 check

Actual result:

Reproduces failure above

Expected result:

Ideally it would build.

The current cargo-audit release (v0.18.1) bumps the proc-macros2 version to one without the failing feature reference, which works with the new chromium toolchain.