opt: specify rust version for nightly

.github/workflows/flutter-nightly.yml

@@ -41,7 +41,7 @@ jobs:
       - name: Install Rust toolchain
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: stable
+          toolchain: "1.62"
           target: ${{ matrix.job.target }}
           override: true
           profile: minimal # minimal component installation (ie, no documentation)