Audit with cargo audit, rather than an Actions plugin (#993)

<!-- The PR description should answer 2 important questions: -->

### What

Same as with `ndc-postgres` and `ndc-postgres-multitenant`, we call a
`just` command in CI, rather than using an actions plugin.

### How

<!-- How is it trying to accomplish it (what are the implementation
steps)? -->

V3_GIT_ORIGIN_REV_ID: 96daa602e4647fdcd641d79539df66a264914efc

v3/justfile

@@ -8,6 +8,12 @@ default:
 build:
   cargo build --release --all-targets
 
+audit:
+  cargo audit
+
+audit-fix:
+  cargo audit fix
+
 format:
   cargo fmt --check
   npx --yes prettier --check .