Spack breaks when installing an environment that contains python + a package that fails to build.

Steps to reproduce the issue

Assume that you have a package "somepackage" that fails to build with spack (in my case this was a one my own python packages that would fail in the setup.py script for some reasons). Do the following to break spack:

$ spack env create myenv
$ spack env activate myenv
$ spack add python
$ spack add somepackage
$ spack install # this will install everything (including python) up to somepackage, which will fail to install, as expected

After this series of commands, all spack commands will fail with a RuntimeError from python indicating:

RuntimeError: Unable to locate python command in /projects/spack/var/spack/environments/myenv/.spack-env/view/bin