Use GitHub macOS ARM runners for building ARM macOS binaries

GitHub introduced free macOS ARM runners on January, and my experience
using them in other projects to improve CI times and be able to actually
run tests on Apple Silicon Macs has been positive. Let's use them in
OxiPNG to hopefully speed up CI a bit, and finally be able to run the
test suite on AArch64 macOS.

.github/workflows/oxipng.yml

@@ -49,9 +49,9 @@ jobs:
           - target: i686-pc-windows-msvc
             os: windows-latest
           - target: x86_64-apple-darwin
-            os: macos-latest
+            os: macos-12
           - target: aarch64-apple-darwin
-            os: macos-latest
+            os: macos-14 # ARM64 runner
 
     env:
       CARGO_BUILD_TARGET: ${{ matrix.target }}
@@ -122,19 +122,11 @@ 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