Remove the abandoned actions-rs actions

The `actions-rs/toolchain` and `actions-rs/cargo` repositories have been
archived on Oct 13, 2023. Those actions are therefore unmaintained.
https://github.com/actions-rs/toolchain
https://github.com/actions-rs/cargo

.github/workflows/ci.yml

@@ -17,21 +17,11 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install Rust ${{ matrix.rust }}
-        uses: actions-rs/toolchain@v1
-        with:
-            toolchain: ${{ matrix.rust }}
-            override: true
-            profile: minimal
+        run: rustup toolchain install ${{ matrix.rust }}
       - name: Run cargo build
-        uses: actions-rs/cargo@v1
-        with:
-          command: build
-          args: --verbose
+        run: cargo +${{ matrix.rust }} build --verbose
       - name: Run cargo test
-        uses: actions-rs/cargo@v1
-        with:
-          command: test
-          args: --verbose
+        run: cargo +${{ matrix.rust }} test --verbose
   formatting:
     name: Cargo fmt
     runs-on: ubuntu-latest