Use Nightly toolchain for docs.rs test build

Was already supposed to use Nightly before, but apparently the `rust-toolchain.toml` file had higher precedence.

Author: Marcono1234

.github/workflows/publish.yml

@@ -35,7 +35,7 @@ jobs:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
       - uses: dtolnay/rust-toolchain@nightly
       - uses: dtolnay/install@cargo-docs-rs
-      - run: cargo docs-rs
+      - run: cargo +nightly docs-rs
 
   publish:
     needs: check-docs-rs-build