Do not run tests for ARM64 macOS targets

It's currently not possible to do so.

.github/workflows/oxipng.yml

@@ -112,11 +112,19 @@ jobs:
           reporter: github-check
           fail_on_error: true
 
+      # There aren't good user-mode ARM64 emulators we can use on x64 macOS hosts.
+      # QEMU doesn't have any plans to add such support due to a lack of kernel
+      # syscall stability guarantees: https://gitlab.com/qemu-project/qemu/-/issues/1682
       - name: Run tests
+        if: matrix.target != 'aarch64-apple-darwin'
         run: |
           cargo nextest run --release --features sanity-checks
           cargo test --doc --release --features sanity-checks
 
+      - name: Build tests (ARM64 macOS only)
+        if: matrix.target == 'aarch64-apple-darwin'
+        run: cargo test --release --features sanity-checks --no-run
+
       - name: Build benchmarks
         run: cargo bench --no-run