CI: don't force arch unless needed

... so that e.g. macOS ARM runners can use a native Julia

However for Julia 1.6 on macOS we need to take special care,
see <https://discourse.julialang.org/t/117019> for details.

Author: fingolfin

.github/workflows/CI.yml

@@ -18,16 +18,25 @@ jobs:
 #          - 'nightly'
         os:
           - ubuntu-latest
-          - macOS-latest
+          - macos-latest
           - windows-latest
         arch:
-          - x64
+          - ''
+        exclude:
+          - os: macos-latest # Apple Silicon
+            version: '1.6'
+            arch: ''
+        include:
+          - os: macos-latest # Apple Silicon
+            version: '1.6'
+            # Install an x86_64 build of Julia, and run it under Rosetta 2 emulation
+            arch: 'x64'
     steps:
       - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v2
         with:
           version: ${{ matrix.version }}
-          arch: ${{ matrix.arch }}
+          arch: ${{ (matrix.julia-arch == '') && runner.arch || matrix.julia-arch }}
       - uses: julia-actions/cache@v2
       - uses: julia-actions/julia-buildpkg@v1
       - uses: julia-actions/julia-runtest@v1