cargo-audit in CI can't actually fail

While looking at CI, I noticed this job had a failing cargo-audit, but actually succeeded. maint/cargo_audit doesn't propagate errors it gets from cargo-audit

CI log extract

$ ./maint/cargo_audit
    Fetching advisory database from `https://github.com/RustSec/advisory-db.git`
      Loaded 404 security advisories (from /usr/local/cargo/advisory-db)
    Updating crates.io index
    Scanning Cargo.lock for vulnerabilities (410 crate dependencies)
Crate:         net2
Version:       0.2.37
Warning:       unmaintained
Title:         `net2` crate has been deprecated; use `socket2` instead
Date:          2020-05-01
ID:            RUSTSEC-2020-0016
URL:           https://rustsec.org/advisories/RUSTSEC-2020-0016
Dependency tree: 
net2 0.2.37
├── miow 0.2.2
└── mio 0.6.23
Crate:         tempdir
Version:       0.3.7
Warning:       unmaintained
Title:         `tempdir` crate has been deprecated; use `tempfile` instead
Date:          2018-02-13
ID:            RUSTSEC-2018-0017
URL:           https://rustsec.org/advisories/RUSTSEC-2018-0017
Dependency tree: 
tempdir 0.3.7
├── tls-api 0.7.0
│   ├── tls-api-test 0.7.0
│   │   └── tls-api-native-tls 0.7.0
│   │       └── arti-hyper 0.2.0
│   ├── tls-api-native-tls 0.7.0
│   └── arti-hyper 0.2.0
└── test-cert-gen 0.7.0
    └── tls-api-test 0.7.