make sure CI fails when we do not recognize the platform

Author: RalfJung

ci.sh

@@ -48,4 +48,7 @@ elif [ "${TRAVIS_OS_NAME:-}" == osx ]; then
 elif [ "${CI_WINDOWS:-}" == True ]; then
   MIRI_TEST_TARGET=x86_64-unknown-linux-gnu run_tests
   MIRI_TEST_TARGET=x86_64-apple-darwin run_tests
+else
+  echo "FATAL: unknown CI platform"
+  exit 1
 fi