pin action to commit hash

.github/workflows/test_bindings.yml

diff --git a/.github/workflows/test_bindings.yml b/.github/workflows/test_bindings.yml
index 8459f9d..c050e0d 100644
--- a/.github/workflows/test_bindings.yml
+++ b/.github/workflows/test_bindings.yml
@@ -49,7 +49,9 @@
           fetch-depth: 0
           persist-credentials: false
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@4cda84d5c5c54efe2404f9d843567869ab1699d4
+        with:
+          toolchain: stable
       - name: Install Ninja (macOS)
         if: runner.os == 'macOS'
         run: brew install ninja