Use the nightly toolchain for the "crater" build

Some crates assume nightly when building the docs when the document-featurues is enabled (eg gix)

.github/workflows/rust.yml

@@ -43,7 +43,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: nightly
       - name: setup patch
         run: |
           echo "" >> ~/.cargo/config.toml