CI: fix security audit (#456)

New Ubuntu images are being deployed, which is causing the cached
binaries not to work due to a GLIBC upgrade:

https://github.blog/changelog/2024-09-25-actions-new-images-and-ubuntu-latest-changes/

To ensure we're caching consistently on the new image, switches from
using `ubuntu-latest` to `ubuntu-24.04`.

We can revert this change after October 30th, when the blog post says
the deployment is complete.

.github/workflows/security-audit.yml

@@ -13,14 +13,14 @@ on:
 jobs:
   security_audit:
     name: Security Audit
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
       - name: Cache cargo bin
         uses: actions/cache@v3
         with:
           path: ~/.cargo/bin
-          key: ${{ runner.os }}-cargo-audit-v0.20
+          key: ${{ runner.os }}-cargo-audit-v0.20-ubuntu-24.04
       - uses: actions-rs/audit-check@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}