[sonic-installer] Import re module (sonic-net#1061)

The import re line was removed from the file in sonic-net#953. However, it is still referenced in the update_sonic_environment() function. Adding it back.

sonic_installer/main.py

@@ -6,6 +6,7 @@
     import configparser
 
 import os
+import re
 import subprocess
 import sys
 import time