Revert "chore(ci): consoldate rust-toolchain calls"

This reverts commit c459bd5.

.github/workflows/build-ffi.yml

@@ -33,10 +33,13 @@ jobs:
       - uses: dtolnay/rust-toolchain@stable
         with:
           toolchain: stable
-          components: rustfmt
       - name: Install shared mime info DB
         if: runner.os == 'macOS'
         run: brew install shared-mime-info
+      - uses: dtolnay/rust-toolchain@nightly
+        with:
+          toolchain: nightly
+          components: rustfmt
       - name: Install doxygen
         if: runner.os == 'Linux'
         run: sudo apt-get install -y doxygen