8bd9527a71
* mpich: add pmi=cray support
After marking `cray-pmi` as external:
```yaml
packages:
cray-pmi:
externals:
- spec: cray-pmi@5.0.17
prefix: /opt/cray/pe/pmi/5.0.17
```
You can now install
```
spack install something ^mpich pmi=cray
```
and
```console
srun $(spack location -i something)/bin/your_app
```