set env var back after test run

Author: sai-sunder-s

system_tests/noxfile.py

@@ -273,14 +273,20 @@ def compute_engine(session):
     # credentials are detected from environment
     is_present = "GOOGLE_APPLICATION_CREDENTIALS" in session.virtualenv.env
     print(f"GOOGLE_APPLICATION_CREDENTIALS is present: {is_present}")
-    del session.virtualenv.env["GOOGLE_APPLICATION_CREDENTIALS"]
+    if is_present:
+        env_var = session.virtualenv.env["GOOGLE_APPLICATION_CREDENTIALS"]
+        del session.virtualenv.env["GOOGLE_APPLICATION_CREDENTIALS"]
     session.install(LIBRARY_DIR)
     default(
         session,
         "system_tests_sync/test_compute_engine.py",
         *session.posargs,
     )
 
+    session.virtualenv.env["GOOGLE_APPLICATION_CREDENTIALS"] = env_var
+
+
+
 
 @nox.session(python=PYTHON_VERSIONS_SYNC)
 def grpc(session):