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 \