fix: watsonx self._project_id not getting set (generative-computing#220)

Author: jakelorocco

mellea/backends/watsonx.py

@@ -96,8 +96,10 @@ def __init__(
             base_url = f"{os.environ.get('WATSONX_URL')}"
         if api_key is None:
             api_key = os.environ.get("WATSONX_API_KEY")
+
         if project_id is None:
-            self._project_id = os.environ.get("WATSONX_PROJECT_ID")
+            project_id = os.environ.get("WATSONX_PROJECT_ID")
+        self._project_id = project_id
 
         self._creds = Credentials(url=base_url, api_key=api_key)
         self._kwargs = kwargs