clear mpicc and friends before each build (#17450)
clear mpicc and friends before each build (#17450)

* clear mpi env vars
iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack