Remove unnecessary toolchain action

The GitHub runners include rustup and a recent stable Rust. We only
need to add the necessary target and we’re good to go.

This removes a lot of warnings because the action used an outdated
GitHub API: actions-rs/toolchain#219

.github/workflows/build.yml

@@ -75,10 +75,7 @@ jobs:
         run: sudo apt update && sudo apt install gcc-aarch64-linux-gnu
 
       - name: Install toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          toolchain: stable
-          target: ${{ matrix.target }}
+        run: rustup target add ${{ matrix.target }}
 
       - name: Build Rust code
         working-directory: ${{ matrix.directory }}