fix(test): switch to nightly toolchain for unstable features

Author: pulsastrix

.github/workflows/ci.yml

@@ -24,9 +24,10 @@ jobs:
       - uses: actions/checkout@v3
         with:
           submodules: true
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@master
         with:
           components: rust-src
+          toolchain: nightly
       - if: matrix.dtls_backend == 'gnutls'
         uses: awalsh128/cache-apt-pkgs-action@latest
         with: