ci: disable persist-credentials in rust.yml

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/rust.yml

@@ -7,6 +7,8 @@ jobs:
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          persist-credentials: false
       - run: ci/set_rust_version.bash ${{ matrix.channel }} ${{ matrix.target }}
         shell: bash
       - run: ci/test.bash cargo ${{ matrix.target }}