- `spack env activate foo`: sets SPACK_ENV to the current active env name
- `spack env deactivate`: unsets SPACK_ENV, deactivates the environment
- added support to setup_env.sh and setup_env.csh
- other env commands work properly with SPACK_ENV, as with an environment
arguments.
- command-line --env arguments take precedence over the active
environment, if given.