ci: add Rust toolchain to update-homebrew job

The post-release job runs `cargo update --workspace` but never
installs a Rust toolchain. The project requires edition 2024 /
rust-version 1.88.0, which the runner's pre-installed cargo
likely doesn't support. Add dtolnay/rust-toolchain@stable.

Author: rursache

.github/workflows/release.yml

@@ -91,6 +91,8 @@ jobs:
         with:
           ref: main
 
+      - uses: dtolnay/rust-toolchain@stable
+
       - name: Update Homebrew formula
         run: scripts/update-formula.sh ${{ github.ref_name }}