Build on macos-11 (#25)

macos-10.15 will be unsupported from 08/30/2022, see :
actions/runner-images#5583

Signed-off-by: Sylvain Leclerc <[email protected]>

Author: sylvlecl

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
           }
           - {
             name: macos,
-            os: macos-10.15,
+            os: macos-11,
             artifact_name: libmath.dylib,
             artifact_path: osx_64
           }