Bugfix for SpackCommand

[rblake-llnl]

I was trying to run something like this using spack-python

bash:
    spack -e ${environment_dir} location -i ${spack_spec}
python:
    location = SpackCommand("location")
    install_tree = location('-i', spack_spec, fail_on_error=True, global_args=["-e", environment_dir])

This fix makes this work as intended.

[wdconinc]

This PR was closed because a rebase resulted in a zero change set: the same fix was merged as part of #17229. I apologize that we were not able to merge your changes earlier, but I encourage you to continue submitting pull requests with fixes.