Use specific nightly build for document CI workflow

`wgpu` takes a long time to document otherwise.

.github/workflows/document.yml

@@ -8,12 +8,12 @@ jobs:
     steps:
     - uses: hecrj/setup-rust-action@v2
       with:
-        rust-version: nightly
+        rust-version: nightly-2023-12-17
     - uses: actions/checkout@v2
     - name: Generate documentation
       run: |
         RUSTDOCFLAGS="--cfg docsrs" \
-          cargo doc --no-deps --all-features \
+          cargo +nightly-2023-12-17 doc --no-deps --all-features \
           -p futures-core \
           -p iced_core \
           -p iced_highlighter \