[build] Don't overwrite user NINJA_STATUS

Author: kastiglione

utils/build-script

@@ -105,8 +105,9 @@ def initialize_runtime_environment():
             'WATCHOS_DEPLOYMENT_TARGET']:
         os.environ.pop(v, None)
 
-    # Set NINJA_STATUS to format ninja output
-    os.environ['NINJA_STATUS'] = '[%f/%t][%p][%es] '
+    # Provide a default NINJA_STATUS to format ninja output.
+    if 'NINJA_STATUS' not in os.environ:
+        os.environ['NINJA_STATUS'] = '[%f/%t][%p][%es] '
 
 
 class JSONDumper(json.JSONEncoder):