try macos-11

Author: cknitt

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
  strategy:
  fail-fast: false
  matrix:
- os: [macos-latest, ubuntu-latest, windows-latest, macos-arm]
+ os: [macos-11, ubuntu-latest, windows-latest, macos-arm]
  ocaml_compiler: [4.14.0]
 
  runs-on: ${{matrix.os}}