pathogens: use top level workflows key

List workflows under a top level `workflows` key in the `nextstrain-pathogen.yaml`
file instead of listing workflows under `compatibility['nextstrain run']`. 

Suggested by @victorlin 
<#462 (comment)>

Author: joverlee521

nextstrain/cli/pathogens.py

@@ -314,9 +314,9 @@ def registered_workflows(self) -> Dict[str, Dict]:
             debug("pathogen does not have a registration")
             return {}
 
-        workflows = self.registration.get("compatibility", {}).get("nextstrain run")
+        workflows = self.registration.get("workflows")
         if not isinstance(workflows, dict):
-            debug(f"pathogen registration.compatibility['nextstrain runs'] is not a dict (got a {type(workflows).__name__})")
+            debug(f"pathogen registration.workflows is not a dict (got a {type(workflows).__name__})")
             return {}
 
         return workflows