ci: cargo-audit fails due to yanked hashbrown 0.15.0

hashbrown 0.15.0 was yanked due to broken support for the borsh codec when the "borsh" feature is enabled. borsh does not appear in arti's Cargo.lock, so arti should be unaffected.

The fix should be as simple as updating the hashbrown version to 0.15.1.

$ ./maint/cargo_audit
    Fetching advisory database from `https://github.com/RustSec/advisory-db.git`
      Loaded 664 security advisories (from /usr/local/cargo/advisory-db)
    Updating crates.io index
    Scanning Cargo.lock for vulnerabilities (613 crate dependencies)
Crate:     hashbrown
Version:   0.15.0
Warning:   yanked
Dependency tree:
hashbrown 0.15.0
└── indexmap 2.6.0
[...]