env: only add `concrete` attribute to abstract specs
env: only add `concrete` attribute to abstract specs

- to aovid changing spec hashes drastically, only add this attribute to
  differentiated abstract specs.

- othherwise assume that read-in specs are concrete

LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack