downgrade macos

So our mac builder is getting killed with sigkill (9).

This is super mysterious:

* rust-lang/cargo#7821
* fortran-lang/fpm#16
* https://github.community/t/github-actions-on-macos-randomly-kill-my-test-program/17387

.github/workflows/ci.yaml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest,   windows-latest,   macos-latest]
+        os: [ubuntu-latest,   windows-latest,   macos-10.15]
 
     steps:
     - uses: actions/checkout@v2