fix: CSP_FRAME_SRC was getting re-assigned instead of being added to

it made sense to re-assign it when the default value was 'none', but
now we will always have at least *.littlepay.com

benefits/settings.py

@@ -312,7 +312,7 @@ def RUNTIME_ENVIRONMENT():
 if RECAPTCHA_ENABLED:
     env_frame_src.append("https://www.google.com")
 if len(env_frame_src) > 0:
-    CSP_FRAME_SRC = env_frame_src
+    CSP_FRAME_SRC.extend(env_frame_src)
 
 CSP_IMG_SRC = [
     "'self'",