ci/github: use explicitly latest stable toolchain

Since the action audit used has rust 1.58, it automatically fails with
strip. If this doesn't work I'll disable the audit cron to stop auto
notification.

.github/workflows/audit.yml

@@ -14,9 +14,15 @@ env:
 
 jobs:
   cargo_audit:
+    name: RUSTSEC Audit
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
       - uses: actions-rs/audit-check@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}