INSTANCE_TYPE is set to regular during the runtime of the post-stop hook of an addon

[Gary.Wang]

To reproduce this,

1. Create an addon that includes the post-stop hook with the following content
```
#!/bin/bash

echo "Current instance type: $INSTANCE_TYPE"
```

2. Bootstrap an application,
 amc app create <app_dir>

3. Check the system log via
 amc logs -f <base_container_id>

Actual result:
  The system log prints out "Current instance type: regular."

Expected result:
  The system log should print out "Current instance type: base."

This is a bug introduced since 1.20.0 release