update-prod-static: Fix incorrect ordering of venv setup code.

We should check if we succeeded in using a venv only after we've
actually setup sys.path to use it.

tools/update-prod-static

@@ -9,10 +9,6 @@ import subprocess
 import argparse
 import sys
 
-# check for the venv
-from lib import sanity_check
-sanity_check.check_venv(__file__)
-
 # We need settings so we can figure out where the prod-static directory is.
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
 import scripts.lib.setup_path_on_import
@@ -20,6 +16,10 @@ os.environ['DJANGO_SETTINGS_MODULE'] = 'zproject.settings'
 from django.conf import settings
 from scripts.lib.node_cache import setup_node_modules
 
+# check for the venv
+from lib import sanity_check
+sanity_check.check_venv(__file__)
+
 parser = argparse.ArgumentParser()
 parser.add_argument('--prev-deploy', metavar='DIR',
                     help='a previous deploy from which to reuse files if possible')