chore(ci): use stable rust for building C API

Author: tottoto

.github/workflows/CI.yml

@@ -190,7 +190,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install Rust
-        uses: dtolnay/rust-toolchain@nightly
+        uses: dtolnay/rust-toolchain@stable
 
       - name: Install cbindgen
         uses: actions-rs/cargo@v1
@@ -204,7 +204,7 @@ jobs:
           RUSTFLAGS: --cfg hyper_unstable_ffi
         with:
           command: rustc
-          args: --features client,http1,http2,ffi -Z unstable-options --crate-type cdylib
+          args: --features client,http1,http2,ffi --crate-type cdylib
 
       - name: Make Examples
         run: cd capi/examples && make client