ci: fix docs workflow by forcing nightly (#218)

Author: Wenzel

.github/workflows/docs.yml

@@ -94,6 +94,8 @@ jobs:
         run: |
           SIMICS_BASE="${HOME}/simics/simics-${{ env.PUBLIC_SIMICS_PACKAGE_VERSION_1000 }}" RUSTDOCFLAGS="--enable-index-page -Zunstable-options" TSFFS_TESTS_SKIP_BUILD=1 cargo doc --document-private-items --workspace --no-deps
           cp -a target/doc/ docs/book/html/crates/
+        env:
+          RUSTUP_TOOLCHAIN: nightly
 
       - name: Set Documentation Permissions
         run: |