Creates an environment in a temporary directory and activates it, which
is useful for a quick ephemeral environment:
```
$ spack env activate -p --temp
[spack-1a203lyg] $ spack add zlib
==> Adding zlib to environment /tmp/spack-1a203lyg
==> Updating view at /tmp/spack-1a203lyg/.spack-env/view
```