CI: switch to dtolnay/rust-toolchain action

actions-rs/toolchain is no longer maintained, and repo is archived.

See https://github.com/actions-rs/toolchain/issues/216

.github/workflows/lynx.yml

@@ -26,9 +26,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install latest Rust stable
-        uses: actions-rs/toolchain@v1
-        with:
-          toolchain: stable
+        uses: dtolnay/rust-toolchain@stable
 
       - id: setup_python
         name: Set up Python ${{ matrix.python-version }} Environment

.github/workflows/python_tests.yml

@@ -24,9 +24,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install latest Rust stable
-        uses: actions-rs/toolchain@v1
-        with:
-          toolchain: stable
+        uses: dtolnay/rust-toolchain@stable
 
       - id: setup_python
         name: Set up Python ${{ matrix.python-version }} Environment