exclude macOS dmd tests

broken due to mir-ion linking issues
https://issues.dlang.org/show_bug.cgi?id=19044

Use LDC instead, it also supports the M1 macs.

Author: WebFreak001

.github/workflows/unittest.yml

@@ -16,6 +16,8 @@ jobs:
         exclude:
         - os: windows-latest
           dc: dmd-latest
+        - os: macos-latest
+          dc: dmd-latest
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2