ci: use persist-credentials: false throughout

We already do this in most of the other Rustls crates, and Zizmor 0.7.0
flags[0] its absence in this repo.

[0]: https://woodruffw.github.io/zizmor/audits/#artipacked

.github/workflows/build.yml

@@ -13,6 +13,8 @@ jobs:
     steps:
       - name: Checkout sources
         uses: actions/checkout@v4
+        with:
+          persist-credentials: false
 
       - name: Install stable toolchain
         uses: dtolnay/rust-toolchain@stable