ci: skip ssh tests on macOS

SSH tests on macOS have begun failing for an unknown reason after an
infrastructure upgrade to macOS 10.13.6.  Disable those tests
temporarily, until we can resolve it.

azure-pipelines.yml

@@ -72,6 +72,7 @@ jobs:
         PKG_CONFIG_PATH: /usr/local/opt/openssl/lib/pkgconfig
         LEAK_CHECK: leaks
         CMAKE_OPTIONS: -G Ninja
+        SKIP_SSH_TESTS: true
 
 - job: windows_vs_amd64
   displayName: 'Windows (amd64; Visual Studio)'