Try to work around toolchain version parsing issue

1.70.0 should be the same as 1.70.

.github/workflows/ci.yml

@@ -80,7 +80,9 @@ jobs:
     - uses: actions/checkout@v4
     - uses: dtolnay/rust-toolchain@master
       with:
-        toolchain: 1.70
+        # NOTE: full version (including .0) to work around
+        # <https://github.com/dtolnay/rust-toolchain/issues/112>.
+        toolchain: 1.70.0
     - name: Check
       # We only run check allowing us to use newer features in tests.
       run: cargo check --all-features