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

@@ -15,9 +15,9 @@ jobs:
           os: ubuntu-latest
         - 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