Install prebuilt cargo-audit to speed up Security Audit workflow (nushell#16615)

The "Security Audit" workflow always builds the `cargo-audit` binary but
it could just use a prebuilt one.

## Release notes summary - What our users need to know

N/A

Author: cptpiepmatz

.github/workflows/audit.yml

@@ -20,6 +20,9 @@ jobs:
     continue-on-error: true
     steps:
       - uses: actions/checkout@v5
+      - uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-audit
       - uses: rustsec/[email protected]
         with:
           token: ${{ secrets.GITHUB_TOKEN }}