Fix Intel Mac CI builds.

It turns out that since GitHub switched the default macOS runners to
aarch64 machines, our CI config has been producing aarch64 binaries both
in the `aarch64-macos` tarballs *and* the `x86_64-macos` tarballs. Oops!

This fixes the config to do the right thing: natively compile aarch64
binaries, and cross-compile x86-64 binaries.

.github/workflows/ci.yml

@@ -13,23 +13,33 @@ jobs:
         include:
         - build: x86_64-linux
           os: ubuntu-latest
+          target: x86_64-unknown-linux-gnu
+        - build: aarch64-linux
+          os: ubuntu-latest
+          target: aarch64-unknown-linux-gnu
         - build: x86_64-macos
           os: macos-latest
+          target: x86_64-apple-darwin
         - build: aarch64-macos
           os: macos-latest
           target: aarch64-apple-darwin
         - build: x86_64-windows
           os: windows-latest
-        - build: x86_64-mingw
-          os: windows-latest
-          target: x86_64-pc-windows-gnu
+          target: x86_64-pc-windows-msvc
     steps:
     - uses: actions/checkout@v2
+    - uses: ./.github/actions/install-rust
+      with:
+        toolchain: stable
+    - run: |
+        rustup target add ${{ matrix.target }}
     - name: Build
-      run: cargo build --verbose
+      run: cargo build --verbose --target ${{ matrix.target }}
     - name: Run tests
+      if: ${{ matrix.target }} == 'x86_64-linux'
       run: cargo test --verbose
     - name: Checking benches
+      if: ${{ matrix.target }} == 'x86_64-linux'
       run : cargo check --benches
 
   rustfmt: