teach spack how to build pmix from git (#16973)
teach spack how to build pmix from git (#16973)

* teach spack how to build pmix from git

* add variant for new REST feature

* reorder package to match guidance
LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack