Enable nightly doc testing again. (#580)

[`pathfinder_simd`](https://crates.io/crates/pathfinder_simd) v0.5.2 is
no longer the latest and Cargo resolves it for `piet-svg` as v0.5.4
which once again works with the nightly Rust toolchain.

Fixes #566.

.github/workflows/ci.yml

@@ -178,14 +178,8 @@ jobs:
  if: contains(matrix.os, 'ubuntu')
 
  - name: install nightly toolchain
- # NOTE: Currently pinned to 2024-02-01 with '@master'. Move to just '@nightly' when possible.
- # Right now that breaks the build due to pathfinder_simd 0.5.2 not compiling with nightly.
- # The issue seems to have been fixed in https://github.com/servo/pathfinder/pull/548
- # However there is no new published crate version containing that fix, yet.
- # See more at https://github.com/linebender/piet/issues/566
- uses: dtolnay/rust-toolchain@master
+ uses: dtolnay/rust-toolchain@nightly
  with:
- toolchain: nightly-2024-02-01
  targets: wasm32-unknown-unknown
 
  - name: restore cache