Fix flux-pmix version for integration tests (#880)

* Update environment-integration.yml

* Update environment-integration.yml

* Update environment-integration.yml

* Update environment-integration.yml

Author: jan-janssen

.ci_support/environment-integration.yml

@@ -10,6 +10,7 @@ dependencies:
 - atomistics =0.3.4
 - cloudpickle =3.1.2
 - flux-core =0.81.0
+- flux-pmix =0.6.0
 - gpaw =25.7.0
 - hatchling =1.27.0
 - hatch-vcs =0.5.0