ci: install Intel C/C++ compiler for full Intel toolchain

Add intel-oneapi-compiler-dpcpp-cpp to provide icx/icpx alongside ifx.
Set all Intel compiler env vars (FC, CC, CXX, MPI wrappers).

Author: sbryngelson

.github/workflows/test.yml

@@ -245,17 +245,20 @@ jobs:
           sudo apt-key add GPG-PUB-KEY-INTEL-SW-PRODUCTS.PUB
           sudo add-apt-repository "deb https://apt.repos.intel.com/oneapi all main"
           sudo apt-get update
-          sudo apt-get install -y intel-oneapi-compiler-fortran intel-oneapi-mpi intel-oneapi-mpi-devel
+          sudo apt-get install -y intel-oneapi-compiler-fortran intel-oneapi-compiler-dpcpp-cpp intel-oneapi-mpi intel-oneapi-mpi-devel
           # Export only new/changed env vars from setvars.sh.
           # `printenv >> $GITHUB_ENV` dumps all vars including shell internals
           # with special characters that corrupt GITHUB_ENV parsing.
           printenv | sort > /tmp/env_before
           source /opt/intel/oneapi/setvars.sh
           printenv | sort > /tmp/env_after
-          diff /tmp/env_before /tmp/env_after | grep '^>' | sed 's/^> //' \
-            | grep -v '^CC=' | grep -v '^CXX=' >> $GITHUB_ENV
+          diff /tmp/env_before /tmp/env_after | grep '^>' | sed 's/^> //' >> $GITHUB_ENV
           echo "FC=ifx" >> $GITHUB_ENV
+          echo "CC=icx" >> $GITHUB_ENV
+          echo "CXX=icpx" >> $GITHUB_ENV
           echo "MPIFC=mpiifx" >> $GITHUB_ENV
+          echo "MPICC=mpiicx" >> $GITHUB_ENV
+          echo "MPICXX=mpiicpx" >> $GITHUB_ENV
 
       - name: Build
         run:  |