the Windows CI runner takes a lot longer than the others; let it do less work

x86_64-apple-darwin is also used as the host OS for the macOS runner, so no need
to test it twice.

Author: RalfJung

ci.sh

@@ -55,7 +55,6 @@ case $HOST_TARGET in
     ;;
   i686-pc-windows-msvc)
     MIRI_TEST_TARGET=x86_64-unknown-linux-gnu run_tests
-    MIRI_TEST_TARGET=x86_64-apple-darwin run_tests
     ;;
   *)
     echo "FATAL: unknown OS"