ci: Enable assertions in CMake build job on Windows

Since commit 5b564c371e (gitlab-ci: add Windows infrastructure,
2020-05-20, v3.18.0-rc1~102^2~1) we build with the `Release`
configuration on Windows to avoid the `-Zi` flag.  However, that also
compiles with `-DNDEBUG` by default, thus suppressing assertions.
Since the build is meant for testing, enable assertions again.