gitlab-ci: pin macOS to a specific Xcode version

.gitlab-ci.yml

@@ -73,6 +73,9 @@
         # Even if we could, it could change if other runners on the machine
         # could run at the same time, so we drop it.
         GIT_CLONE_PATH: "$CI_BUILDS_DIR/cmake ci"
+        # TODO: Factor this out so that each job selects the Xcode version to
+        # use so that different versions can be tested in a single pipeline.
+        DEVELOPER_DIR: "/Applications/Xcode-11.3.app/Contents/Developer"
 
 .macos_ninja: &macos_ninja
     extends: .macos