Faster CI for cargo audit tests

[Shnatsel]

Speeds up CI by:

1. Running cargo test without the --release flag, avoiding compiling the code twice (tests are run with cargo run without --release)
2. Using the sparse index instead of fetching crates.io index via git, which would cause a regression in test performance (gix and its deps such as miniz_oxide compiled in debug mode)

The Rust toolchain is bumped to v1.71 for sparse index support but MSRV is still enforced by the workspace job.

[Shnatsel]

cargo-audit / test-linux execution time drops from 13m 44s to 5m 24s.

cargo-audit / test-macos drops from 17m 15s to 8m.

cargo-audit / test-windows drops from 26m 34s to 13m 16s.

[Shnatsel]

rustsec / test on MSRV down from 14m 36s to 8m 6s