fix list

we were checking for preferences, but they were not set. checking for sources is more reliable.
secondly, the isfile check needs a full path

Author: doniks

ubports-qa

@@ -104,11 +104,10 @@ def list_exists(branch):
 def list_lists():
     return [
         f.split("ubports-")[1].split(".list")[0]
-        for f in os.listdir("/etc/apt/preferences.d")
-        if os.path.isfile(f) and f.startswith("ubports-")
+        for f in os.listdir("/etc/apt/sources.list.d/")
+        if os.path.isfile("/etc/apt/sources.list.d/" + f) and f.startswith("ubports-")
     ]
 
-
 def add_list(branch):
     if list_exists(branch):
         return