ci: Fix macOS build by downgrading to 10.15

.github/workflows/build_and_test.yaml

@@ -38,7 +38,10 @@ jobs:
     needs: lint
     strategy:
       matrix:
-        os: ["ubuntu-latest", "macos-latest", "windows-latest", "self-hosted-linux-arm64"]
+        # NOTE: macos-10.15 is required for now, to work around issues with our
+        # build system.  See related comments in
+        # .github/workflows/custom-actions/build-packager/action.yaml
+        os: ["ubuntu-latest", "macos-10.15", "windows-latest", "self-hosted-linux-arm64"]
         build_type: ["Debug", "Release"]
         lib_type: ["static", "shared"]
         include:
@@ -47,7 +50,7 @@ jobs:
             target_arch: x64
             exe_ext: ""
             build_type_suffix: ""
-          - os: macos-latest
+          - os: macos-10.15
             os_name: osx
             target_arch: x64
             exe_ext: ""