workflow: Switch back from macOS-latest to 10.15 for now

See actions/virtual-environments#4060 - the 11.0 image is crashing for us so we'll upgrade when it's more mature

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
     if: "!contains(github.event.head_commit.message, '[skip ci]') && !contains(github.event.head_commit.message, '[ci skip]')"
     strategy:
       matrix:
-        os: [ubuntu-18.04, windows-2016, macOS-latest]
+        os: [ubuntu-18.04, windows-2016, macOS-10.15]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v1