TEST: Try Macos CI with Big Sur

Author: hkratz

.github/workflows/main.yml

@@ -139,7 +139,7 @@ jobs:
         - target: aarch64-unknown-linux-gnu
           os: ubuntu-latest
         - target: x86_64-apple-darwin
-          os: macos-latest
+          os: macos-11
         - target: x86_64-pc-windows-msvc
           os: windows-latest
         - target: i686-pc-windows-msvc