Remove macos-11.0 environment

See #2989 (comment)

The environment currently is not reliable. Maybe because it's a preview only according to actions/runner-images#1814 ?

Author: amai2012

.github/workflows/CI-unixish.yml

@@ -9,7 +9,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-20.04, macos-10.15, macos-11.0]
+        os: [ubuntu-18.04, ubuntu-20.04, macos-10.15]
       fail-fast: false # Prefer quick result
 
     runs-on: ${{ matrix.os }}