Ignore RUSTSEC-2019-0003.

[jsirois]

We have a fix in-flight in
pantsbuild/rust-protobuf#2 that will still need
this --ignore even when we're consuming it. Adding the --ignore now
silences nightly CRON audit noise in the meantime and going forward
until we can upgrade to a public official release of protobuf with the
RUSTSEC-2019-0003 fix.

Part of fixing #7760

[jsirois]

Tested locally with:

$ ./build-support/bin/ci.sh -a

[=== 00:00 CI BEGINS ===]
[=== 00:00 Setting interpreter constraints to ['CPython==3.6.*'] ===]
[=== 00:00 Running cargo audit on rust code ===]
    Fetching advisory database from `https://github.com/RustSec/advisory-db.git`
      Loaded 24 security advisories (from /home/jsirois/.cache/pants/rust/cargo/advisory-db)
    Scanning /home/jsirois/dev/pantsbuild/jsirois-pants/src/rust/engine/Cargo.lock for vulnerabilities (318 crate dependencies)
     Success No vulnerable packages found
[=== 00:06 CI ENDS ===]


SUCCESS