Change CI config to have separate cuda and cxx standards

.jenkins/cscs/env-gcc-cuda-11.sh

@@ -4,7 +4,8 @@
 # Distributed under the Boost Software License, Version 1.0. (See accompanying
 # file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
-cxx_std="17"
+cxx_std="20"
+cuda_std="17"
 gcc_version="9.3.0"
 boost_version="1.75.0"
 hwloc_version="2.0.3"
@@ -15,6 +16,7 @@ spack_arch="cray-cnl7-haswell"
 spack_spec="pika@main arch=${spack_arch} %${spack_compiler} +cuda malloc=system cxxstd=${cxx_std} ^boost@${boost_version} ^hwloc@${hwloc_version} ^cuda@${cuda_version}"
 
 configure_extra_options+=" -DPIKA_WITH_CXX_STANDARD=${cxx_std}"
+configure_extra_options+=" -DCMAKE_CUDA_STANDARD=${cuda_std}"
 configure_extra_options+=" -DPIKA_WITH_MALLOC=system"
 configure_extra_options+=" -DPIKA_WITH_CUDA=ON"
 configure_extra_options+=" -DPIKA_WITH_EXAMPLES_OPENMP=ON"