Fix #108: Disable macOS 10.15 runner for build-m2 action temporarily (#106)

- Disable MacOS 10.15. temporarily for m2 build in GitHub Actions

Author: ndoschek

.github/workflows/build.yml

@@ -8,7 +8,7 @@ jobs:
       # do not cancel other builds if one fails
       fail-fast: false
       matrix:
-        os: [windows-2019, ubuntu-18.04, macos-10.15]
+        os: [windows-2019, ubuntu-18.04]
 
     runs-on: ${{ matrix.os }}
     timeout-minutes: 20