Bugfix: respect order of mirrors in mirrors.yaml (#13544)
Bugfix: respect order of mirrors in mirrors.yaml (#13544)

Commands like "spack mirror list" were displaying mirrors in a
different order than what was listed in the corresponding mirrors.yaml
file.

This restores commands to iterate over mirrors in the order that
they appear in the config file.
LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack