Fix envconfig unit test

[dhiltgen]

No description provided.

[mxyng]

There are a few other places where os.Setenv is being used in tests:

server/envconfig/config_test.go|11 col 2| os.Setenv("OLLAMA_DEBUG", "")
server/envconfig/config_test.go|14 col 2| os.Setenv("OLLAMA_DEBUG", "false")
server/envconfig/config_test.go|17 col 2| os.Setenv("OLLAMA_DEBUG", "1")
server/sched_test.go|24 col 2| os.Setenv("OLLAMA_DEBUG", "1")
integration/max_queue_test.go|31 col 3| os.Setenv("OLLAMA_MAX_QUEUE", fmt.Sprintf("%d", threadCount))
integration/basic_test.go|52 col 9| defer os.Setenv("OLLAMA_MODELS", oldModelsDir)
integration/basic_test.go|54 col 8| err = os.Setenv("OLLAMA_MODELS", modelDir)