Use insert for all sys.path manipulations

Closes #6867.

devscripts/fish-completion.py

@@ -6,7 +6,7 @@ import os
 from os.path import dirname as dirn
 import sys
 
-sys.path.append(dirn(dirn((os.path.abspath(__file__)))))
+sys.path.insert(0, dirn(dirn((os.path.abspath(__file__)))))
 import youtube_dl
 from youtube_dl.utils import shell_quote