Revert "ci: Remove macos-11.0 as it is unreliable"

Author: Noah Gorny

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
   bats-test:
     strategy:
       matrix:
-        os: [ubuntu-20.04, ubuntu-18.04, ubuntu-16.04, macos-10.15]
+        os: [ubuntu-20.04, ubuntu-18.04, ubuntu-16.04, macos-10.15, macos-11.0]
 
     runs-on: ${{ matrix.os }}