Make start_kernel env extend os.environ (#859)

* Make start_kernel env extend os.environ

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

jupyter_server/services/sessions/sessionmanager.py

@@ -1,6 +1,7 @@
 """A base class session manager."""
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
+import os
 import pathlib
 import uuid
 
@@ -274,7 +275,7 @@ async def start_kernel_for_session(self, session_id, path, name, type, kernel_na
         kernel_id = await self.kernel_manager.start_kernel(
             path=kernel_path,
             kernel_name=kernel_name,
-            env={"JPY_SESSION_NAME": path},
+            env={**os.environ, "JPY_SESSION_NAME": path},
         )
         return kernel_id