Feat/add host envs (#3808)

* add guide for dev builds

* hosted env file for openmm-8 beta

* explain how to host an env

* add a namespace to support different os

* pin all the versions

* add version pins and env name

* Added pytest

devtools/hosted-envs/openmm-8-beta-linux.yaml

-name: openmm-8-beta
+name: openmm-8-beta-linux
 
 channels:
   - conda-forge/label/openmm_rc
@@ -7,6 +7,7 @@ channels:
 
 dependencies:
   - openmm==8.0.0beta
-  - openmm-ml
-  - openmm-torch
-  - nnpops
+  - openmm-ml==1.0beta
+  - openmm-torch==1.0beta
+  - nnpops==0.2
+  - pytest

devtools/hosted-envs/openmm-8-beta-mac.yaml

-name: openmm-8-beta
+name: openmm-8-beta-mac
 
 channels:
   - conda-forge/label/openmm_rc
@@ -7,5 +7,6 @@ channels:
 
 dependencies:
   - openmm==8.0.0beta
-  - openmm-ml
-  - openmm-torch
+  - openmm-ml==1.0beta
+  - openmm-torch==1.0beta
+  - pytest