workflow: Use macOS 10.15 for now for CMake builder

.github/workflows/ci.yml

@@ -40,7 +40,7 @@ jobs:
           eigen: NO
 
         - profile: macos-eigen-coverage-unity-xcode
-          os: macOS-11
+          os: macOS-10.15
           config: Coverage
           unity: YES
           generator: Xcode
@@ -50,7 +50,7 @@ jobs:
           eigen: YES
 
         - profile: macos-nometa-standard-makefile
-          os: macOS-11
+          os: macOS-10.15
           config: Standard
           unity: NO
           generator: Unix Makefiles