Source devnull in environment_after_sourcing_files (closes #15775) (#15791)
Source devnull in environment_after_sourcing_files (closes #15775) (#15791)

spack.util.environment_after_sourcing_files compares the local
environment against a shell environment after having sourced a
file; but this ends up including the default shell profile and
rc, which might differ from the local environment.

To change this, compare against the default shell environment,
expressed here as 'source /dev/null'.
LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack