Using `sys.executable` to run Python in a sub-shell doesn't always work in a virtual environment as the `sys.executable` Python is not necessarily compatible with any loaded spack/other virtual environment.

- revert use of sys.executable to print out subshell environment (#14496)
- try instead to use an available python, then if there *is not* one, use `sys.executable`
- this addresses RHEL8 (where there is no `python` and `PYTHONHOME` issue in a simpler way