Update C++ version to 20 in nightly workflow (#314)

Co-authored-by: Yuuichi Asahi <[email protected]>

Author: yasahi-hpc

.github/workflows/build_nightly.yaml

@@ -32,7 +32,7 @@ jobs:
               c: gcc
               cxx: g++
             cmake_flags:
-              cxx_standard: 17
+              cxx_standard: 20
               kokkos: -DKokkos_ENABLE_OPENMP=ON
               kokkos_fft: -DCMAKE_CXX_FLAGS="-Wall -Wextra" -DCMAKE_COMPILE_WARNING_AS_ERROR=ON
           - name: threads
@@ -50,7 +50,7 @@ jobs:
               c: gcc
               cxx: g++
             cmake_flags:
-              cxx_standard: 17
+              cxx_standard: 20
               kokkos: -DKokkos_ENABLE_SERIAL=ON
               kokkos_fft: -DCMAKE_CXX_FLAGS="-Wall -Wextra" -DCMAKE_COMPILE_WARNING_AS_ERROR=ON
           - name: cuda
@@ -68,7 +68,7 @@ jobs:
               c: hipcc
               cxx: hipcc
             cmake_flags:
-              cxx_standard: 17
+              cxx_standard: 20
               kokkos: -DKokkos_ENABLE_HIP=ON -DKokkos_ARCH_VEGA90A=ON
               kokkos_fft: -DCMAKE_CXX_FLAGS="-Wall -Wextra -Werror"
           - name: rocm
@@ -88,7 +88,7 @@ jobs:
             cmake_flags:
               # building for Intel PVC was unsuccessful without the proper
               # device, so for now, we simply generate generic Intel GPU code
-              cxx_standard: 17
+              cxx_standard: 20
               kokkos: -DKokkos_ENABLE_SYCL=ON -DKokkos_ARCH_INTEL_GEN=ON
               kokkos_fft: -DCMAKE_CXX_FLAGS="-Wall -Wextra"
     steps: